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Home / Basic Electronics / Object counter using LDR and calculator

Object counter using LDR and calculator


To make an object counter we can use a simple dark detector circuit and a normal calculator. Pole and NO (Normally Open) terminals of a relay have to be connected to the two pins which are used to activate ‘=’ key of calculator.
As we block the light falling on LDR, the calculator starts counting.
For the calculator to work as a counter, we first need to press ‘1’ and ‘+’ keys.

All calculators may not work as counter, you need to check with the following steps:

a. First press 1

b. Then Press +

c. Then keep on pressing =

Each time when you press “=”, the calculator should increase the numbers and display 1,2,3,4… and so on. If your calculator displays “1″ every time you press “=”, then your calculator cannot be used as counter.

In our circuit, we use relay to connect two wires of ‘=’ key that eventually increases numbers on calculator.


We have recently released a 3 digit digital object counter kit. You can buy the kit at BuildCircuit store. 
Step 15- Connect a 9V battery and your kit is ready Buy now button

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  1. nice project….i also did it and it was awesome only i had to look out for a dark place as the ldr sensitivity was a slight of a problem.

  2. how to disable auto power off feature in an electronic calculator because most of the calculators nowadays are electronic….I am hoping for your response…thank you in advance

  3. You said that in the calculator we have to press “=” sign after pressing 1 then in normal calculator wlif do this thing we didn’t need any circuit,,,,,,,

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