Need some help with a trigger/timer circuit for LED

[DCS]

Hello! I'm putting together a bullet-impact signal based on a successful design already deployed by other persons. I'm a novice, so I need some help.

Goals: reliable, rugged, some redundancy, inexpensive, utilize existing materials.

Overview: Hardened steel gong placed at 200 to 500 yards. Piezo vibe sensor attached to back of target or to target framework. 20 foot cable extends from vibe sensor to LED fixture. When the gong is struck by a rifle bullet, the LED fixture lights up for 3 seconds, then goes off, resets, and awaits next hit. I know how to protect various components from bullet strikes or fragment strikes. What I need help with is the electronics.

The system is used in a competition environment where multiple observers are staring at the target waiting for the light signal to indicate a hit.

My original signal design involved an array of LED stars with associated driver circuits and optics. Then I tested an Amazon "7w 300lm mini cree" $4 flashlight at 500 yards in bright sunlight. Bingo. When set to narrow beam, they are perfect for what I need. It was not difficult to aim them such that the beam was reliably oriented at the observation station. Each $4 includes an emitter, driver, and optic. I've owned 11 of these lights for almost 3 years, using 14500 batteries that have been reliable for me for over 5 years. I like 'em.

I plan to bundle three of them in a tube that is oriented at the observation station.

My sensor circuit is a "cking" variation of a published circuit. It is in use and has proven successful at switching a motorized contraption when a bullet impact is detected.

As shown, this circuit has a switchable choice of different resistors. This permits use for pistol vs rifle and also for clusters of targets near each other. Selecting the proper setting prevents a hit on one target from triggering nearby sensors, etc. I won't be using that switchable array. I only need one set value, because my targets are all rifles targets, and each is located far from the other.

My goal is to bundle the three flashlights in a tube without batteries. I want to solder a +wire to the center of the back of each flashlight driver board, and a - wire to each chassis or perhaps to something in the clicky switch of each light. Another option would be a dummy battery to bridge between the driver and clicky, and make both wire connections in the clicky apparatus. Each flashlight then will have its own pigtail that junctions with the main cable. If one light goes down, the other two will still function.

My power source would be three 14500 batteries in parallel.

Things I don't know:
1 - Where to put the battery pack. Twenty feet of cable is a long run. The sensor package load is small compared to the LED load. But the transistor switch is in the sensor package. This is a perfect example of a design issue that i just don't know enough to address. Does 20' even matter? Do I need two independent 2-conductor runs from the battery? If I use that approach, does the switched LED load affect the sensor pack function, causing some kind of loop problem whenever the LED comes on line?

2 - Can I safely solder a lead to the back of each driver, or would it be better to use a dummy battery with a thru-connector and make both connections in the clicky area?

3 - I haven't done a current test on the flashlight yet, but three of them bundled must pull at least 2 amps, if not more. This is way outside the CD74HC123 limits. Do I just add another bigger transistor that has a direct hot connection, and let the signal from the chip trigger the 1st transistor which then trips the 2nd? I need some help on the design here.

4 - I'm still trying to figure out the re-triggering and pulse width capabilities of the 74HC123 chip. I'm starting to think that the 74 chip alone can't give me a 3-second "on" interval followed by reset. So maybe I need to put a 555 between the 74 and the transistor. I'll be doing some more reading, but feel free to jump in and steer me in the right direction.

Well, I hope this makes sense and I'm somewhat on the right track. If you have one of these flashlights, get a couple of radios and a friend and go see what I'm talking about. For an observer who is staring at the flashlight 500 yards away waiting for it to come on, there is no question when it happens. Amazon even sells a green emitter model now, but twice the price. I'm gonna start with my white ones, maybe try green later.

Thanks in advance for helping me get this thing together.

DCS

 

[Steve K]

some thoughts....

- how far away is the target? If it is 500 yards away, then I might see the need for a few watts of light. Otherwise, I'd be tempted to use a handful of 5mm LEDs with a tight beam.

- regarding the problem of getting a 3 second pulse... Linear Technology does make some nice modern timer circuits that can provide long time constants. Check their "timer blox" line of products:
http://www.linear.com/products/timerblox#

- If you do use a 555, make sure that it is the CMOS version, the LMC555. The bipolar version will kill that button cell battery rather quickly.

- you may be able to replace the bipolar junction transistor, BC547, with a logic level mosfet that can handle quite a few amps. I'd use a small cap from gate to source to keep it from turning on and off too quickly. That will just cause inductive spikes that could damage the mosfet.

 

[DCS]

Thanks Steve. We tested the flashlights. They work great at 500 yds.

My battery pack will be parallel 14500's.

I'll check out the timer blox.

What I need help with is the circuit design downstream of the 74 chip.

The diagram above switches an independently powered a/c motor.

I need to revise it so that it switches 3.8v to the LEDs from a shared power source (14500 battery pack).

Thx.

 

[DIWdiver]

1. I would put the batteries in the lights, remove the rubber button from the tailcap, and solder wires across the switch. That way you don't need dummy cells or battery packs, and you can still make them back into flashlights.

2. Yes you can solder to the driver, but I wouldn't (see above).

3. Yes, you need a second, bigger transistor. Something like this would perform nicely and be easy to work with: http://www.digikey.com/product-detail/en/IRLIZ44NPBF/IRLIZ44NPBF-ND/811964 I would put it with the lights to keep the high current paths short so you don't have to worry about what kind of wire you use and there won't be enough inductance to create spikes that could damage the transistor. Any 2-conductor wire you have would work to connect the trigger circuit to the light section.

4. The circuit as shown should give a pulse around 2.2-2.5 seconds. I'd increase R3 to 150K, that should give you pretty close to 3 seconds, maybe a bit over.

To run it this way you'd need to modify the trigger circuit by moving the E terminal from the battery negative to the battery positive.

At the lights, the FET source connects to the negative side of the switch(connects to the battery spring), the drain goes to the positive side of the switch(connects to the case). You need a resistor from gate to source. A 22K would work well with the 1K mentioned below. Otherwise you could use 10K. The trigger circuit connects with the E terminal to the gate and the C terminal to the source.

That's what you need to make it run. As an added precaution against static electricity, I'd add a few more things, especially 2 and 3:
1. A 1K resistor between T1 and the C terminal
2. A 5.1V zener diode with the cathode (band) to gate and anode to source.
3. A 100 ohm resistor between the input and the gate.

 

[Steve K]

4. The circuit as shown should give a pulse around 2.2-2.5 seconds. I'd increase R3 to 150K, that should give you pretty close to 3 seconds, maybe a bit over.

oh heck.. I didn't even look at the component values in the schematic.. I just looked at the datasheet. I gotta agree; bumping up R3 ought to do pretty well.

 

[DCS]

Incredibly helpful. Thanks for taking the time for such a comprehensive reply!

 

[spencer]

My electronics aren't the best either but I would probably use an Arduino for this entire project.

I don't know how many you need to make but it would probably bring the project cost up. If it is only one, I would say the ease of use would outweigh the cost.

 

[DCS]

Thanks for your thoughts spencer. Need to build 3 kits. Gonna stick with the basic circuit.

I found digikey's scheme-it app, which brought me along to my current plan (and bottleneck), and also helped me waste a few hours.

Credit to TK Hareendran for the initial circuit design and to cking for testing and revisions that tune and adjust sensitivity of the sensor.
R2 tunes the sensor roughly to the 500-900Hz anticipated frequency of bullet impacts. R1 & R5 desensitize the sensor so that nearby impacts won't trigger a response.
I anticipate that I will have to test and adjust variations of these values to get the circuit running as desired.

I took DIW's guidance as a starting point, learned more about transistor switches, then thought more about physical location of components and power supply, then reverted back to the battery-pack concept. The flashlights are the simplest affordable reliable means for achieving a narrow-beam hi-power LED with built-in heatsink and driver, so cutting a few up is cheaper and easier than building similar signal lamps.
I'd also like to have a single power source.
Lastly, I'd like to keep everything possible in a centralized container that can be placed behind physical protection.

So my next set of questions is about basic layout. Below is the same basic circuit with a focus on piezo and battery. Ignore transistors for now.

The piezo is affixed to a steel target, connected by wires to a circuit package located ~18' distant from the target.
The circuit package (CP) contains the chip, the transistors, and a battery pack.
The flashlight signal tree will be connected to the CP by a ~2' lead. Total system length is 20' from target to signal tree, with CP placed as close as possible to the signal tree.

Questions:
1 - a piezo generates a high voltage upon disruption. Can I locate it 18' from the CP?

2 - if so, would it be best to locate R2 at the piezo, or at the CP? I understand that the 18' run will have a resistance of it's own, which may suggest a lower final value for R2.

3 - for R1 & R5, same question as above: is there a reason why they won't work if located 18' from the piezo?

4 - The battery will be two or three 14500 in parallel. Considering the above layout as a starting point, am i on the right track to have a centralized battery that can drive the circuit and the signal lamps?

I'll put up a separate post about transistors.

Thanks!

 

[DIWdiver]

1. The piezo voltage will be some volts to some tens of volts. If you are interested in frequencies under 1000 Hz. there's no problem putting that on 18' of cable.

2&3. I doubt it will make any difference where you place the components. I would put them all in the CP just for simplicity. The cable will have a resistance not more than a few ohms, which will be trivial compared to the high resistances you have in the circuit. The cable will also have some capacitance, though at 18' that should also be trivial.

4. Yes, the battery can run both the lights and the CP.

 

[DCS]

It's astounding how much I've learned online in the last 15 years just by bumping into people like you! Many thanks!

For the next drawings, I used a transformer symbol to represent the buck/boost driver in the flashlights.

...

What about something simple like the next drawing. For an N-ch mosfet, does the chip output signal have to exceed Vcc to trigger it? If not, maybe something like this can work?

What about using a P-ch mosfet? If I'm reading the 74HC123 data sheet correctly, the pizeo voltage spike can cause a voltage drop signal on output pin 4. It looks like the drop is from Vcc to less than half a volt. Is that enough to "turn on" a P-ch mosfet like this one: http://www.digikey.com/product-detail/en/IRLIB9343PBF/IRLIB9343PBF-ND/811932 ?
I'm still trying to grasp Vgs(th).

Whatever the eventual outcome, I'm grateful for your advice on static and reverse protections, and for your help in getting this thing running.

Once the design is finished, I'll order parts and teach my son how it works on a breadboard. Then we'll build one and test it at the range, etc.

 

[DIWdiver]

Use the second circuit. The only reason to use two transistors is to have one at each end of the long line.

For reasons having to do with the physics of semiconductors, P-channel FETs are inherently not as good as N-channel ones. There are times where their opposite 'polarity' is enough advantage that we use them anyway, but this circuit would work equally with either. So stick with N-channel.

I assume that's an IRF-something-44N? Good choice.

 

[DCS]

Great!

I haven't chosen a mosfet yet. You originally linked the IRLIZ44NPbF, which I loosely referenced in the drawing.

Presently I'm learning more about datasheet specs, and I've been browsing the IR selection of mosfets.

I also discovered that maybe a simple Darlington might work? TP120 family? Maybe with a diode across it.

For my resistors, will 1/4watt suffice, or will I need higher-rated anywhere?

I deleted the unused drawing. Never know when somebody will just grab something online, so no sense leaving it up.

 

[DIWdiver]

Oh, I forgot that I mentioned that part.

What's simpler than a single FET? You won't find anything that will work better. A darlington will probably drop 0.7-1.0V, maybe more depending on your choice. What's a TP120? I couldn't find anything that looked remotely useful.

1/4W will be fine.

Agreed on the drawing.

 

[DCS]

Done! mosfet it is!

I'll post a diagram and results once finished.

 

[DCS]

1. The piezo voltage will be some volts to some tens of volts. If you are interested in frequencies under 1000 Hz. there's no problem putting that on 18' of cable.
...
The cable will have a resistance not more than a few ohms, which will be trivial compared to the high resistances you have in the circuit. The cable will also have some capacitance, though at 18' that should also be trivial.
...

Here's a snip from a random post above. It's an example of "including how's and why's" in an answer. There must be 20 examples of that in this thread. Incredibly helpful.

On this end, we're gonna breadboard this thing and run it and check current in a few places and do some wattage calcs, and just basically learn more while we're doing.

I cannot tell you how helpful all those "how's and why's" are in terms of advancing our understanding and our education in basic electronics. Sometimes electronics can be simple. Sometimes it can be a mind-boggling information overload for a noob. Having someone tell you whether your totally off track is a great help in early stages. Having them explain how's or why's is a huge bonus.

Many thanks DIWdiver!

 

[DIWdiver]

You're very welcome. It's a pleasure conversing with someone who really wants to learn.

 

[DCS]

We tried this circuit today. It didn't work. We had an unexpected result.

I added a 3.3v zener to protect from possible huge voltage spikes from the piezo.
My perception: this zenr will provide 2 functions: it will clip the input signal voltage from the piezo, and it will also drain away negative voltage signals from the piezo.

Question 1 - Is this correct?

*************

I didn't place a load resistor between Vcc and Drain, because the loads are complete flashlights with built-in buck/boost drivers, so I don't need to limit the current.

Question 2 - Correct?

(For the circuit test, I only have 1 flashlight connected).

*************

I used an older F15NO5L logic level transistor (I had some laying around). It has very similar specs to the IRLIZ44NPbF.

My main problem: when I supplied power to the circuit, the LED came on immediately, drawing 450mA between Vcc and the load.

It was as though the FET was immediately saturated.

My method for connecting power was to just briefly touch the ammeter lead to the positive leg of the breadboard. I pulled it away instantly when the LED came on.

So I disconnected the piezo and tested the circuit again. Same result. The LED flashes on if I touch the hot ammeter probe to the breadboard.

I did this several times and apparently killed the FET.

Bummer.

I checked all my components and connections (again), and everything was correct.

So I've been reading about mosfet switches and trying to determine whether I had a problem related to the gate resistor: ie too much or not enough resistance.

My latest perceptions from reading online:
1 - mosfet's have near-zero current from gate to source when saturated.
2 - My chip datasheet suggests that the pin 13 signal voltage should be very close to Vcc, and that it's current limits are +- 25mA.
3 - My mosfet data sheet describes a Idss of 1 - 50 micro-amps and a Igss of 100 nano-amps.

Question 3 - Are these the current levels at which the mosfet switching begins to occur?

4 - Some online authors suggest that chips are poor mosfet gate drivers due to frequency issues, but I can't get my head around that.
5 - I want to try to calculate an exact gate resistor value, but apparently this isn't done for mosfets in the same way as it is done for BPT's, because mosfets don't have a gate-source current. ???

It seems like this circuit should be simple: a chip voltage signal closes a mosfet switch for a certain interval. But when I tried it, the mosfet was never open, and it died after several "touches" of the ammeter probe. When it was working, Id never exceeded 450mA.

Question 4 - I'm a bit confused.

 

[DCS]

I tested all my transistors. They test good.

Discovery! They don't turn off when the signal voltage is removed. You have to place a resistor from gate to ground to bleed them off when the signal is removed.

I'll try it...

 

[DCS]

Well the pull down resistor didn't change anything. I tried a 10K and a 100K.

I'm starting to wonder about power supply. Pin 16 and 3 are exposed to the rail, but the chip days they have a 5mA o rless current limit. Do i need resistance there?

Baffled by this thing...

 

[PeterH]

It's probably a good idea to debug the control circuit without the output attached. Less chance of smoking a component that way.