Become a subscriber (Free)

Join 29,000 other subscribers to receive subscriber sale discounts and other free resources.
Don't worry -- your e-mail address is totally secure. I promise to use it only to send you MicroZine.

Electronic Fish Scale

This project, an electronic fish scale from 1989, shows how to make a simple set of fish weighing scales.

These scales are still based on the movement of a spring and use a potentiometer attached to the spring movement to detect the weight of a fish.

It is an interesting design because it takes an existing system (spring scales) and digitizes them in a very simple manner.

Although it is a simple design the output is linear since the spring output is linear (See Figure 10) unless it gets over-stretched!.

Executive Summary of the Electronic Fish Scale

An electronic fish scale having a spring connected to a hook to which the fish or other item to be weighed is attached. Attaching the weight to the hook causes linear displacement of the spring. A rigid coupling is attached to the spring and connected to a variable resistor such that movement of the spring results in a change in the effective resistance of the variable resistor.

Electronic circuitry is interconnected with the variable resistor such that a signal is generated which is indicative of the effective resistance of the variable resistor. The generated signal is sampled and processed at a microprocessor having stored data corresponding to the weight equivalent to the variance in the variable resistor-dependent signal.

A display receives a signal output from the microprocessor and displays and weight measured.

Background of the Electronic Fish Scale

Traditional fish scales include scales indicating the weight of the fish to the user through the physical displacement of the spring. The user directly reads the weight corresponding to the displacement off of the scale connected to the spring. More recently, fish scales have been developed which include electronic circuitry.

However, electronic scales currently on the market are believed to have a limited range of weights which are capable of being measured and displayed, have a limited number of features, and use relatively bulky and expensive components.

Summary of the Electronic Fish Scale

There is provided in accordance with the present design an electronic fish scale comprising a spring means for generating linear response to a force imparted thereto, such as a fish to be weighed; a variable resistor; and a displacement indicating means rigidly connected to the spring means for displacement therewith in response to said force, said displacement indicating means further connected to the variable resistor such that the amount of change in the effective resistance of the variable resistor is substantially proportionate to the amount of physical displacement of the displacement indicating means.

The scale further comprises an electronic circuit means for generating an electrical signal substantially indicative of the effective resistance of the variable resistor; and display means for converting the electrical signal generated by the electronic circuit means into a display substantially indicative of the force imparted to the spring means.

Figure 1 : Is a top, front and right side perspective view of the scale in accordance with the present invention for the electronic fish scale

Description of the Electronic Fish Scale

FIG. 1 shows an electronic fish scale 10 comprised of front and back molded plastic sections 12 and 14, respectively. The housing of the fish scale 10 comprises an upper display and control section 16, a middle handle section 18 and a lower base portion 20. The handle section 18 forms ridges and recessed 22 for improving the grip on the scale. The scale housing is watertight and generally floatable, and substantially encloses the spring and electronic circuitry.

Figure 2 : Is a front elevation view thereof for the electronic fish scale

FIG. 2 shows the LED display 30 of the electronic fish scale 10. In a preferred embodiment, the display 30 will indicate the unit of measurement, shown as pounds in FIG. 2 by the designation "LBS" 110. By activating the KG/LBS button 34, the display screen may be shifted between the display in pounds and a display in kilograms.

The numerical display of the weight includes a pounds display 112 and an ounces display 114. When kilograms are displayed, the larger-sized designation 112 indicates kilograms, and the smaller-sized designation 114 indicates hundredths of a kilogram.

When the TOTAL WEIGHT button 36 is activated, the display 30 will display a total weight equalling the sum of one or more prior measurements taken by the fish scale. This mode is designated in the display by the "TW" indication 116.

The upper display portion 16 of scale 10 includes additional control buttons, including the "big fish" control 32, weight button 38, enter button 40, and on/off button 42, explained in further detail below. A looped end portion 50 of hook 48 used to hold the fish or other item to be weighed is exposed at the bottom of the scale 10.

Figure 3 : Is a rear elevation view thereof for the electronic fish scale

FIG. 3 shows the back side of fish scale 10. The hook 52 for weighing the fish is shown in the stored position. The hook is held in place by a retaining member 56 which is attached to the fish scale housing. The hook is retained by the retaining member 56 within a recess formed in the housing.

The recess includes a substantially linear portion 48 for retaining the neck of the hook, and a wider upper portion 120 for receiving the curved portion of the hook. The use of the recess and the retaining member secures the fish hook in a safe location.

The hook 52 is attached by a pivotable chain length 54 held in recess 122 formed in the base portion of the housing. The chain length 54 is connected to the spring (not shown) located within the housing. The hook 52 is sized appropriately for hooking and holding the fish or other item to be weighed.

The fish scale 10 also includes a compartment cover 60 for covering a compartment within the housing suitable for holding a tape measure. The cover may be removed by pressing on the cover detaching plate 58. The tape measure may be removed from the compartment for use, or may be mounted such that the tape measure end 124 is exposed on the outside of the housing, allowing use of the tape measure while the tape measure is secured within the compartment. The end of the tape measure 124 is located adjacent to hook 52 so that the length of the fish held on hook 52 may be easily determined.

Figure 4 : Is a right side elevation view thereof for the electronic fish scale

Figure 5 : Is a left side elevation view thereof for the electronic fish scale

FIG. 4 and FIG. 5 show side views of the scale 10. The retaining member 56 is shown slightly extending from the housing. The recesses 120 and 48 for holding the fish hook are indicated. Also shown is the curved opposite end of fish hook 52 connected by chain length 54 (not shown) to loop 50 connected to the spring scale (not shown).

Figure 6 : Is a top plan view thereof for the electronic fish scale

FIG. 6 shows a bottom view of the scale 10 with the tape measure compartment cover 60 removed. Removal of the tape measure compartment cover reveals a recess 130 for receiving the tape measure. The housing forms a slot 132 for receiving the end of the tape measure, indicated in FIGS. 3 and 7 at 124.

Also shown is compartment cover 60 detached from the scale housing, including retaining projections 62,64. When the cover is attached to the housing, the retaining projections are received at recesses 66,68 of the scale housing.

Figure 7 : Is a bottom plan view thereof for the electronic fish scale

FIG. 7 shows a bottom view of the scale 10 with the compartment cover 60 in place, including the exposed end of the tape measure 124. The tape measure compartment cover 60 extends to the bottom of the housing as shown. Also shown is chain length 54, which links the bottom of the fish hook 52 to loop 50.

To use the fish scale, the on/off button 42 is pressed (see FIG. 2). The display will be activated, indicating whether kilograms or pounds are being displayed, and the most recent measurement will be displayed. To weigh a fish, the hook 52 is released from the retaining member 56 such that the hook hangs down below the scale housing. In the preferred embodiment, the display will automatically turn off after a few seconds if the scale is not used.

Once the hook is released and ready for weighing, the fish (or other item to be weighed) is hooked on the hook 52 and hung from below. The weight button 38 is then pressed, resulting in a display of "OOoo". Once the weight button 38 is depressed, the display 30 will display a limited number of interim readings of the weight of the fish until a final reading, signified by an audible beeping sound, is stored. That final value will be displayed in display 30. The value may be converted between kilograms and pounds by pushing the KG/LBS button 34.

To save an accumulated total of weights, for example, for a number of fish, the ENTER button 40 is pressed. Pressing the enter button will result in a display of an accumulated total of the weights selected to be added to the total weight sum. The display of the total weight is signified by the "TW" display 116 in screen 30.

To display the total weight without entering the currently-displayed weight, the TOTAL WEIGHT button 36 is pressed. Pressing either the ENTER or TOTAL WEIGHT buttons will result in a temporary display of the total weight, with the display returning to the most-recently-recorded weight within a few moments.

The fish-shaped emblem on button 32 represents the "big fish" feature. By pushing button 32, the weight of the largest fish (or other item) is displayed. Display of this value is signified by the fish-like display 118 shown in FIG. 2 within display 30. This display is also temporary, with the display returning to the most recently measured weight within a few moments.

If none of the buttons are activated within a few seconds, the display turns off automatically. However, the most recent weight may be redisplayed simply by activating the on/off button 42. The on/off button 42 may also be pressed to turn the display off.

Figure 8 : Is a schematic diagram of the electronic circuitry in accordance with the present invention as integrated with the spring for the electronic fish scale

View larger image here.

FIG. 8 is a diagram showing a portion of the electronic circuitry of the electronic scale. A variable resistor R1 is shown comprised of resistances R3 and R4. As shown in greater detail below, the spring used to measure the weight of a fish in accordance with the present design will cause the variable resistance values R3 and R4 to change.

Typically, the variable resistance will be a sliding resistor. In a preferred embodiment, the resistor will be selected such that it will have a relatively long resistive element, such as 45 millimeters. Such a resistor will be able to handle, for example, all values outputted by a spring which indicate a linear movement of up to 35 millimeters when a weight of up to 30 pounds is applied.

Use of such elements will allow a display of up to 30 pounds with a resolution of about 1 ounce. The accuracy of the scale is approximately plus or minus 5%.

The weight W260 applied to spring 200 will create a displacement of rigid coupling 202 rigidly connected to arm 254 of spring 200 according to the formula F=kX. The rigid coupling 202 will be located at a position designated Xo when no weight is applied to the spring. When a weight W is applied to the spring, the rigid coupling 202 will be displaced to the location designated Xw.

This displacement will result in a corresponding displacement XR of the conductor 204 slidingly engaged with variable resistor R1, XR is equal to the difference between Xw and Xo. The displacement XR increases the length of resistor R4 and increases the value of the resistance of R4. The displacement results in a corresponding reduction in the length of resistor R3 and reduction in the value of the resistance of R3. R4 is connected in series between variable resistor R2 and the timer 220.

R3 is connected in series between capacitor C1 and timer 220. The spring is interconnected with resistor R1 such that the value of resistor R3 is substantially linearly proportionate to the weight of the fish measured by the spring.

A typical value of the variable resistor R1 is 500,000 ohms. For resolution of one ounce on a total scale of 30 pounds, this resistance corresponds to approximately 1,000 ohms per ounce. By way of example, C1 is chosen to be a one microfarad capacitor and the sampling rate by the microprocessor is approximately 0.1 ms. R2 is a variable resistor provided to compensate for variations in the spring.

The timer 220 is a voltage controlled oscillator, for example a 555 IC. The timer 220 is responsive to a total resistance RT (R1 plus R2), which is a constant, as well as the varying resistance R3. Capacitor C1 is selected such that a suitable time constant is obtained for both the total resistance RT and resistance R3 for appropriate processing by the timer 220.

The time constant corresponding to the total resistance of the circuit, T1, equals 0.693.times.RT .times.C1. The time constant corresponding to variable resistance R3 is designated as T2 and is equal to 0.693.times.R3 .times.C1.

Figure 9 : Is a diagram representing the output wave form of the voltage-controlled oscillator in accordance with the present invention for the electronic fish scale

A wave is outputted by the timer 220 having a high state having a substantially constant duration T1, and a low state having a variable duration T2 proportionate to R3. An example of such a wave form is shown in FIG. 9. The total period of this wave, a square wave as outputted by timer 220, is equal to the sum of T1 and T2 and is designated as T.

Since T1 is proportionate to the total resistance RT (R1 plus R2) and T2 is proportionate to R3, the following is true:

The signal output by timer 220 is received by the four-bit microprocessor 210. A typical integrated circuit device suitable for this use is the LC 5733. The microprocessor 210 is programmed to repeatedly sample the output of the timer 220 for a predetermined period of time. During this sample period, the microprocessor counts the number of "low" readings and the number of "high" readings output from the timer 220.

As the timer output is counted by the microprocessor, the number of low-value samples within a time period and the number of high-value samples within the same timer period are stored. This sampling will generate values indicative of the time periods T2 and T1.

T1 will depend on predetermined and constant values RT (equal to R1 +R2) and C1. A typical value of T1 is 310 ms. That value will remain essentially constant when the scale is in use. The value of T2 varies with the weight applied to the spring. A typical value for T2 when no weight is applied to the spring is 40 milliseconds. A typical value for T2 when a weight of approximately 30 pounds or 13.57 kilograms is applied to the spring is 230 ms.

Figure 10 : Is a table indicative of the calibration data stored in accordance with the invention; and for the electronic fish scale

The value of T2 will vary between the minimum and maximum values (40 ms. and 230 ms. in the example cited) in a substantially, but not necessarily completely, linear form. However, to determine the correct weight from the value of T2, the signal is processed and the correct weight determined from calibration data stored in microprocessor 210 which correspond to a deliar calibration curve of corresponding values as shown in FIG. 10.

The left scale of the calibration curve indicates the calibration number which corresponds to a signal calculated from processing of the signal output from the timer 220 and is proportionate to T2 and R3. The horizontal scale indicates the weight in ounces applied to the spring which corresponds to the calibration number.

The values of the table are stored in the microprocessor 210. The microprocessor is programmed such that the proper value for the weight may be determined and outputted from the microprocessor to the display 230. The six key control panel 240 provides the electrical interconnection between controls 32, 34, 36, 38, 40 and 42 and the microprocessor 210.

It should be further noted that the ratio of T2 to T1 is directly proportionate to the ratio of R3 to RT, and thus any variation in the tolerance or value of the circuit components will not affect the ratio. This improves the accuracy of the measurement system.

In determining the weight applied to the scale, the microprocessor is programmed to respond to the difference in the value of T2 when the weight to be measured is applied to the spring and the value of T2 when there is no weight applied to the spring. The weight is proportionate to this difference divided by T1, which is also proportionate to the difference between the value of R3 when the weight to be measured is applied to the spring and the value of R3 when no weight is applied to the spring, divided by the total resistance RT.

The microprocessor 210 counts the number of low-value samples outputted by timer 220 by detecting the transition and the output wave form from timer 220 from a high-value state to a low-value state. The sampling may be performed by comparing the current count with a previous count to determine equality between the counts, or by the fixed number iterations sampling the output wave from the timer.

The low-value count when no weight is applied is subtracted from the stabilized low-value count when the weight is applied to the string to obtain a normalized low-value count. That normalized value is divided by the high-value count.

This normalized and divided value is compared to the calibration table data stored in the microprocessor to determine the appropriate binary coded decimal output signal to be outputted to the display 230 such that the weight or other values is correctly displayed. A light emitting diode display or a paper printer may also be used for the display.

The microprocessor is electrically interconnected to the control buttons collectively identified as control panel 240, and including keys 32, 34, 36, 38, 40 and 42 described elsewhere.

The conversion table between values of R3 and corresponding weights may be assembled by subjecting the spring to successive controlled weights and reading the value generated by the microprocessor which corresponds to the normalized low-value count divided by the high-value count of the timer wave output.

The generation of such a table obviates the need for additional scaling for parameters such as the spring constant, since these factors are already taken into account.

The microprocessor has the ability to store values in the off state. The processing program and the conversion table are stored in read only memory in the microprocessor. Random access memory in the microprocessor is used for working storage. This random access memory is used for storing the total weight measured, the most recent weight measured, and the largest weight measured.

The largest weight measured is recalled by pushing the corresponding button (32 in FIG. 2). The total weight measured, a sum of selected individual weights previously measured, is displayed by pushing button (36 in FIG. 2). The most recent weight measured is displayed automatically when the on/off switch 42 is activated, and is displayed automatically a few moments after the total weight button 36 or the maximum weight button 32 are activated.

Pressing the ENTER key 40 causes the displayed weight to be retained and added to the total weight stored and displayed when total weight button 36 is activated. Pressing the enter key also causes the displayed weight to be compared to the maximum weight stored and displayed when button 32 is activated.

In an alternative embodiment of the present design, a different variable resistor is used such that the effective range of the weights measured by the scale may be increased. The length of variable resistor RT will limit the range of weights measurable by the scale. The conductive element 204, shown in FIG. 8, cannot be extended beyond the ends of variable resistor RT. However, the displacement of the conductor 204 which may be measured by the circuitry is increased if a multi-turn variable resistor gear arrangement is used, as shown in FIG. 10.

Figure 11 : Is a side view of a multi-turned variable resistor gear arrangement capable of indicating displacement in accordance with the present invention for the electronic fish scale

Displacement of the spring arm 254, FIG. 11, will displace rigid coupling 202' and will correspondingly displace the gear tooth arm 310. Displacement of the gear tooth arm 310 results in rotation of the gear wheel 320. This rotation is transferred by belt 330 to a multi-turn potentiometer 340 which converts the linear motion of arm 310 into a changed resistance RP, which is processed as R3 in the embodiment using the sliding-scale variable resistor described above.

Click here for more project ideas.

New! Comments

Have your say about what you just read! Leave me a comment in the box below.

Claim Your: Useful

"Arduino Software Guide"

   Right Now...

Jump from the electronic fish scale page to
Best Microcontroller Projects Home Page.

Privacy Policy | Contact | About Me

Site Map | Terms of Use


Visit our Facebook Page:

   Click Here

Recent Articles

  1. Using the MAX6675 or How to easily measure Extreme temperatures

    How to use the MAX6675 and an Arduino to measure temperatures from 0°C to 1024°C with two components: A chip - the MAX6675, and a Sensor: - a type-K thermocouple.

    Read more

  2. How to use the DHT22 (or DHT11) with an Arduino; full code and description. Also including a comparison of the DHT11 vs DHT22.

    The Essential Guide to the DHT22/11 humidity sensor including a library code guide for the Arduino. Learn how to determine Humidity, Dew point and Heat Index.

    Read more

  3. How to Use the MAX7219 to drive an 8x8 LED display Matrix on the Arduino.

    The Essential Guide to the MAX7219; What it is and how you can easily use one in any of your projects.

    Read more

Sign up for MicroZine
''The'' Microcontroller Newsletter

Enter your first Name and primary email address in the form below:

And receive absolutely FREE a full project for:

"Measuring Analogue Voltages
Without An ADC"

(Using only one pin).

Instant Download:
You Can
Get It Right Now

Warning: This project could be  Removed 
at any time.  

It will  NOT be 
available indefinitely SO
To avoid 
disappointment  get it:


Don't worry -- your e-mail address is totally secure. I promise to use it only to send you MicroZine
Remember this is a project with full description and fully debugged C Source code - and it's not available from the main website.

You can only get it through this newsletter.

To get exclusive access Enter your first name Name and primary email address Now in the form above.:

But wait !

There's more...

You'll receive more
free and exclusive reports as well as site information and site product updates

Scroll up to the form above and sign up NOW. Don't forget it's FREE and if you don't like it, you can unsubscribe at any time.

Click Here Now to use the form above to get your Valuable information absolutely free.

Readers Comments

"I wanted to thank
you so so so much
for all the information
you have provided in
your site it's


- Ranish Pottath

"This site really is
the best and my favorite.
I find here many useful
projects and tips."

- Milan


"Awesome site,
very, very easy and nice
to navigate!"

- Matt

Learn Microcontrollers

"Interested in

Sign up for The
Free 7 day guide:


"I am a newbie to PIC
and I wanted to say
 how great your
site has been for me."

- Dave


"Your site is a great
and perfect work.

- Suresh


"I couldn't find the correct
words to define
yourweb site.

Very useful, uncovered,
honest and clear.

Thanks so much for
your time and works.

- Anon