Liner Counter


Introduction: Liner Counter

About: I am a photographer, a tinker, an electronics technology engineer, and author; I write short stories and poetry for the love of writing. I started writing poetry in high school over thirty years ago where I ...

 In the questions and answers forum at Instructables a fellow DYI member asked about building a liner counter. He was building a Coil Gun and he wanted a way to track the number of times he fired his Coil Gun. He wanted six LEDs, two green, two yellow, and two red LED, and he wanted them to turn off one at a time as a projectile passes between an IR LED and a Phototransistor.

I came up with a modified Johnson Counter, the counter and added the IR LED and Phototransistor to the clock.

Step 1: Modified Johnson Counter

As in most circuit simulator programs, what works in the program does not necessarily work in the real world. Since I have over 100,000 components at my disposal I offered to build and test the circuit on a bread board.

Step 2: This Is the Finished Circuit

I built and tested this circuit a number of different ways using SN74LS74 and SN74LS175 ICs, With SN74LS175 ICs the circuit is smaller and uses 2 ICs less and half the wires than building it with SN74LS74 ICs.

This is two different ways to wire the circuit.

It works the way they wanted, turn on the circuit and all the LEDs turn on, if all 6 LEDs do not come on press the reset button and all the LEDs will turn on.

Every time you pass an object between the IR LED and the Phototransistor, one LED turns off at a time until all 6 LEDs are off, then when you press the reset button all 6 LEDs turn on.

If you pass an object between the IR LED and the Phototransistor a 7th time or more and the LEDs start to turn on.

Step 3: Parts

1. Proto board

1. IR or UV LED

1. IR or UV Phototransistor

1. CJ78M05 voltage regulator

1. Push button switch

1. 220 uF 16 volt electrolytic capacitor

1. 10 uF 16 volt electrolytic capacitor

2. SN74LS175N D Flip Flops

3. 1N4001 Diode

6. LEDs, 2 red, 2 green, 2 yellow

9. 1 KΩ ¼ watt Resistors

If you need a place to get the ICs you can get them at

For the voltage regulator just enter 78M05 in the search box.

You can get the datasheets at

Step 4: Upping the Design

I always like to improve on things and I thought the color order could be a bit confusing. I changed the color order to four green, two yellow, and one red, for plenty ammo, green, low on ammo, yellow, and out of ammo or reload, red.

I think the next counter I design and build will be a numerical counter.



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You sure know your stuff. If I sat down to do something like this.. it wouldn't end well!

1 reply

That is the fun part of electronics, Toasting components.

in the parts list there are only 2 D flip flops used, how come that in the schematics there are 8?

1 reply

There are only 2 flip flop ICs because each of the ICs have 4 d flip flops in them making a total of 8.


can I use a photodiode or another IR LED instead of a phototransistor?

1 reply

I never tried it but as long as it will transfer enough power it should work.

I'm also wondering what is the essence of the 8th D flip flop you used? it doesn't look like that it has any effect on the circuit.

1 reply

ok the schematic in step 4 is the one I should use if I want the red LED to be my indicator that there is no ammo left in the coil gun right?

1 reply

Yes that is right.

In that circuit I used a SN74LS175N D type flip flop.


and what flip flop did you use here?is it D flip flop?

1 reply

I used D type flip flop and tested it in SN74LS74 and SN74LS175.

But any flip flop you can make a Johnson Counter with will do.


sir may I ask what is the difference between those two schematics in step 2? thanks

1 reply

Just call me Joe

The top one is positive logic.

The bottom one is negative logic.