The project is a bullet counter for an airsoft rifle. Got a G36C rifle for fun, and inspired by Aliens I wanted to have a rifle with a bullet counter too! Though my little project came across a little snatch regarding power consumption. Wondering if anyone can shed light into the subject.

I've put together the following components:

- IR Emitter and sensor to detect bullet leaving barrel
- 74HC14N to stabilize IR sensor's trigger and send TTL signal to counter
- counter circuit 74LS90 and 74LS47 (times 3 digits) to drive 7 segment LED display
- 5V regulator to take power from 8.4v main battery pack


The simple 5v power supply that I used, power drawn from the main battery which drives a motor when a shot is fired


IR sensor, pin 2 of the 74HC14N is the counter's input


The LED counter consist of:
R1-R7 7 470 Ohm 1/4 Watt Resistor
U1 1 74LS90 TTL BCD Counter IC 7490,74HC90
U2 1 74LS47 TTL Seven Segment Display Driver IC

The circuit worked great. Problem comes when I fire a shot and power is drawn from the main battery from a power hungry motor, the load drops just enough to render the circuit useless. The display turns off or comes back with a random number.

An attempt to make things work I disconnected the 5v regulator and put together 4 Ni-Cad batteries to drive the counter unit, but as it turns out the circuit consumes quite a lot of power and the tiny Ni-Cad battery pack ran flat in 10 minutes.

I wanted to ask for any suggestions as to what I can do to make everything work.

- Shall I hook the main battery (plus 5v regulator) and the Ni-Cad in parallel with diodes?
- Would the LED be the ones that are draining power? Should I take the power from the main battery instead and let the Ni-Cad power the chips?
- Is there a way to find out why my circuit is eating up so much power?

It's rather surprising that the circuit would use so much power. I can actually feel the 74LS47 and the 74LS90 being warm during operation. Considering that these chips are supposed to be low power, I found it kind of strange, but being a novice I don't know how to check for power consumption.

That's it! Hope anyone can shed light into my silly little project. Thanks in advance. [/img]

With no circuit to go on diagnosis is a bit difficult,

Have you fitted current limiting resistors to the
7 seg display or connected it direct to the counter?

I suspect you have not and this could be the root of your problems.

Isolate the counter from the motor with a low ohm resistor
and stabilise the logic supply with an electrolytic capacitor.
That should stop it reseting when the motor load drops the
voltage down.

Final suggestion use CMOS chips and lose the 5v reg

Just updated the original post. I've found in another post that mentioned a counter which only uses one CMOS IC to drive every digit... wish I had found that post earlier. :roll: But then the CMOS requires a common cathode 7 segment LCD display and those were much harder to find at the scrap electronics parts store.

Originally Posted by tansis

Isolate the counter from the motor with a low ohm resistor
and stabilise the logic supply with an electrolytic capacitor.
That should stop it reseting when the motor load drops the
voltage down.

Final suggestion use CMOS chips and lose the 5v reg

Thank you for the suggestion. There is already a capacitor at the power supply, is that close to what you were suggesting or should I consider something else?

And would increasing the resistance on R1 to R7 lower power consumption?

Lastly, because the IR circuit and the LED still require 5v, I don't think I have much alternatives, but to keep the regulator. If anyone can shed light in that area it'll be much appreciated.

Thanks again!

well you could do a few things, use modulated IR, low power regulator, flash the display leds and so on. If you went with an LCD it would use much less power but would be harder to see and take more development...

Thank you for all your suggestions. At this point I have two power source in my rifle - the main battery and the small Ni-Cad battery. I'm planning to have the main battery power the LED display while the Ni-Cad would power the 74LS90 and 74LS47 plus IR sensor units. Can that be done with my circuit below?



Using CMOS to act as the counter also sounds like an interesting alternative. The only CMOS LED counter that I found requires LED displays with common cathode. Just curious, is that my only alternative? (just so I don't have to ditch my current LED setup)

Thanks again!

I dont think I saw this mentioned and since power consumption is a concern, consider also building an efficient buck converter for your power supply. Low cost, few parts, not much space required. 8-18V input no problem unlike the wasteful 7805.

Thanks for the very useful replies!

At this point I've tried feeding the 5V supply to the 5v leads on the LED while the counter circuits take a seperate power from external battery with no success. Somehow because the two batteries share the same ground, the counter got messed up while shots are fired. Numbers on the counters are now jumping back and forth. Probably from the drop/change in current from the main battery affecting the 74LS47. The strict power requirement on the 74ls IC's seems to be the main reason of why things aren't working...

So looks like I should just drop the project and look for another solution involving CMOS that is much less strict about having power supplied at 5v. The only concern I have right now is that I'm still using the following sensor:



And I'm afraid that when voltage drops, the circuit would just stop working. Guess I wouldn't know until I try!

Big thanks to jbeng for your recommendations. I'm starting to look into the datasheets now.

There is a major problem with the 7 segment LED counter (using 74LS90 TTL BCD counter IC & 74LS47 TTL seven segment display driver IC). I have built one but it start at 1 rather than 0. I have also tried building another same one and this time it start with 5. I have tried to combine the two circuit and i am getting 51 but i want it to start at 00 :? . Is there anyone able to solve my problem.. hope to get a reply as soon as possible.