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