Hello, I'm working on a project that involves shooting a target using a rubber band gun. The target will have two LED, one of them will be on at any time. The two LEDs will toggle when I hit on target. I am using a piezo that is stucked on to the target to detect a hit. I use an op-amp to try to increase the sensitivity of the target.

With my little knowledge in electronics, I've come up with a schematic below, but it does not work when I implemented it. Any help would be appreciated! Thanks.

 

Two problems.

1) 74LS76 is not designed to run off 6V supply. Use a 4000 series CMOS flip-flop instead.

2) Q1 is wired incorrectly. Connect 10k resistor from Vcc to the collector, collector to the CLK input of the flip-flop.
Connect the emitter to GND.

 

If you reference both the inverting input and the non-inverting inputs of the LM318 to 0V, then pin4 of the opamp will have to be supplied with a voltage that is -3V or more negative (dual, split supplies).

There are opamps available (e.g. LM358) that can have their inputs at 0V even if the Vss pin is at 0V, too.

Mr Chips beat me to the other problems...

 

In a recent test with BB & blow pipe, A 555 worked. See post by Jesse Klein, best way to detect impact on a paper target, post # 83

 

Thanks guys! I do not have a 4027 and a lm358 with me now. So I've tesed using 4.5V battery for the following circuit and it worked. I'll test it out again once I get those two components.

However, I have another issue now. It seems that there are some noise? When I tap the target, the LEDs sometime toggled twice.

 

You could use a spare 4027 flip-flop as a monostable to overcome the noise problem, then clock U3A with its output. Like this:

Why aren't the two outputs of U3A driving two LEDs?

 

Is that a three-wire bicolor LED?. 4.5V is almost too low for the opamp. Can you bump it up to 6V (four cells)?

 

The 4.5V was just to test the 7476 that I have.

It didn't work when it was running on 6V though. One LED was always switched on and the other LED flickers when I tap the target.

Is that a three-wire bicolor LED?. 4.5V is almost too low for the opamp. Can you bump it up to 6V (four cells)?

Yea that's a bicolour LED, I wanted to use one for the actual circuit, but I'm currently testing on two normal LEDs because I don't have them yet.

Anyway, there is a change in plan: instead of toggling two LEDs, I need to light up only one LED for a few seconds. This is the circuit that I have:

(don't have a lm358 yet)

A minor problem is that every time I close S1, the LED will light up for some time. Why is this?

 

If you reference both the inverting input and the non-inverting inputs of the LM318 to 0V, then pin4 of the opamp will have to be supplied with a voltage that is -3V or more negative (dual, split supplies).

There are opamps available (e.g. LM358) that can have their inputs at 0V even if the Vss pin is at 0V, too.

Mr Chips beat me to the other problems...

Why is it necessary for the Vss of an LM318 to be negative and not 0V?

 

Why is it necessary for the Vss of an LM318 to be negative and not 0V?

Opamps have a property called "input common mode range", which is listed on their data sheet. It is a characteristic of the LM318 that both of its inputs must be kept at a minimum of ~3V more positive than the Vss pin. Look it up.

If you bias the inputs to 0V (as you have done), then the Vss pin must be -3V or more negative than the inputs.

The input common mode range of an LM358 extends to -0.5V with respect to Vss, so Vss can be grounded.

 

Insert a retriggerable monostable multivibrator circuit set for 10-100ms between the output of U1 and R3 (U2 and Q1 in the second circuit).
This should solve both problems of double triggering and constant output. Then you can go back to the dual LED solution.

 

The pin 2 of a 555 is a comparator so using another comparator in front is redundant. Use a high impedance V divider to adjust bias or low Z pot & a large R between tap & p2. Something like attachment with piezeo connected from ground to pin 2. Use LED instead of SCR. Adj. R3 & C1 to suit. R1 should be 1.5 to 2 M ohm so that center of pot about 1/3 Vcc.

 

More like your wishes: With 2 cm piezoe disc on paper target, output is about 2V damped wave. Found it a bit dificult shooting rubber bands at target while monitoring scope.

 

Thanks Bernard, I've tested your first circuit with the following schematics. It makes sense that another comparator is redundant. However, I'm quite sure that my earlier circuit(with the additional comparator) made the target quite a bit more sensitive base on a bit of testing and my gut feeling.

How can I make the target more sensitive with this circuit?

 

I've test your second circuit with some modification due to the lack of components. Likewise, it is not as sensitive as the one with an additional comparator. I had to increase the stretch of my rubber band by 2cm in order to light up the LED.

Anyway, all the circuits that I have tried which involve the 555 timer has an issue where the LED would light up momentarily when I connect up the batteries. Why does that happen? Does it happen to you too?

More like your wishes: With 2 cm piezoe disc on paper target, output is about 2V damped wave. Found it a bit dificult shooting rubber bands at target while monitoring scope.

What does C1 do?

 

C1 is to supress high freq. noise, 1nF might be too large, drop C1.
You have no bias adjustment on pin 2, R2 is a pot. On your print, R4 should be 2 X R5 , better values in 100k range like 150k-300k with 50k pot. Bias should be adjustable from 1/3 Vcc to about 1/2 Vcc. Bypass pin 5 to gnd. with .1 uF, I mostly ignore p5 but for narrow V range of signal, a quiet ref V is needed.
If triggering on power-up remains a problem,remove connection to p4 & replace with 100k, Vcc to p4, bypass with 1 uF p4 to gnd.

 

I assumed that piexoe was high Z but how high? Paralled piezoe with 2.5 M pot & continued to shoot & reduce down to 10k range; replaced pot with 100k & repeated. Cutoff at about 12k, so bias divider in 100k range looks good.
At what distance are you shooting? My rubber bands go about 30 ft, trigger target at about 10 ft. Tests were at 5 ft.

 

Thanks for the explanation. I'm not that good with electronics, so would you please explain why there need to have a bias adjustment at pin 2?

From what I have read about 555 timer, a voltage lower than 1/3 Vcc at pin 2 would make the output high and anything higher than 1/3 Vcc would make the output low. So wouldn't that mean that, as long as my resistance allow for a voltage greater than 1/3 Vcc at pin 2, the circuit would be fine?

My rubber band can go about 15 ft. My test target is a plastic ferrero rocher box that is 12.5x9.5 ft, and I sticked a 27mm piezo at the centre. When I test my shots at 7 ft away, some shots that landed further from the piezo would not be detected. Btw, what would be a good material to stick my piezo on and how should I stick it so that it would allow the target to be more sensitive?

I'll try with the pot across the piezo some time later. Thanks Bernard.

 

I'm not suggesting a pot in parallel with sensor. Just put the 50k pot where I show it on schematic as a sensitivity control. For mounting sensor, in a 2 in square of thin cardboard, cut an 18 mm hole & spot glue sensor over hole. Square attached to bottom of targer with paper clips. Your target seems awfully large for rubber band bullets. I would use a tatget no more than 12 in X 18 in suspended from 4 corners with rubber bands. You can omit R6 & C3 if you do not have problems with triggering at power-up.
Note: R 4 should be 75k, forgot to subtract top half of R2.

 

Opps, the target is 12.5x9.5 in, not ft. Typed wrongly there. Anyway, thanks a lot for the help Bernard.

I've managed to increase the sensitivity by adjusting the voltage at pin 2 to be as close as 1/3 Vcc. The R6 & C3 has also helped to resolve the initial power-up issue. Anyway, why does the timer trigger the LED at the start when I power up the circuit?