Creating a "One Shot" laser trigger.

[rhd]

I'm trying to create a "one-shot" trigger for a laser driver that would essentially close a circuit to a relay for 1/5th of a second, and then open the circuit again. On the other side of the relay would be driver circuit itself.

Now there are some other ways I could accomplish this, but I want that "physical click" of the mechanical relay, and I like the look of the components involved in this approach. I'm going to us a 555 circuit, with big ol ugly aluminum caps

I'm hoping someone can take a peak at the circuit I'm using, and my resistor choices. It has been about 10 years since I've built a 555 circuit (made some homemade PCBs with a laser printer to create a timelapse SLR trigger about a decade ago)

I'm going to use this circuit:

With the 1Meg resistor replaced by a 200K resistor, and all other values retained. My understanding is that this will create a ~200 ms "ON" state, followed by a permanent "OFF" state, no matter how long I keep the 555 circuit closed.

What I am not totally clear on, is whether the circuit will trigger a second time if I open the circuit, and then close it ("fire it") again. It seems to me like the second time I close the 555 circuit, nothing would happen. Am I right? If so, that's not desirable.

 

[Meatball]

Nope, the 555 only goes HIGH as a function of the voltage at pin 2. So once the output has been triggered, the 555 does its job by only firing once, and then staying low until the voltage at pin 2 goes low (below 1/3 Vcc) again. I think you're on the right track.

Do you have in mind what kind of driver you will be triggering and how you will use the 555 output to control it?

 

[rhd]

Oh interesting, ok.

It's going to trigger a mechanical relay, which will close, and then power up a 1084 based 4.5A linear circuit

 

[Benm]

Nope, the 555 only goes HIGH as a function of the voltage at pin 2. So once the output has been triggered, the 555 does its job by only firing once, and then staying low until the voltage at pin 2 goes low (below 1/3 Vcc) again. I think you're on the right track.

Isnt that what the leftmost 100k resistor does? It discharges the capacitor (such that both ends are at Vcc) after you release the switch, so it should trigger again if you press the switch again.

Its been a while since i played with 555s, but just connect a led and resistor to the output to see what it does. Also, a 555 will not drive a mechanical relay directly, you need to add a transistor and quenching diode to make that work.

 

[bobhaha]

Edit: Nevermind... misread the post.

 

[jcranmer]

Hmm...It's been since high school since I used a 555 timer. Depending on cost and size issues I would use an Arduino like board connected to a relay.

I've been playing around with a Teensey 2.0 ($16) for a little while now and I don't think it would take much to get it to do what you want.

 

[lasersbee]

Hmm...It's been since high school since I used a 555 timer. Depending on cost and size issues I would use an Arduino like board connected to a relay.

I've been playing around with a Teensey 2.0 ($16) for a little while now and I don't think it would take much to get it to do what you want.

A 555 and the associated parts will cost about $1.00 for that circuit.
You don't need a $16.00 MCU for such a simple circuit..

@rhd...
you are on the right track... just test the circuit on a breadboard..


Jerry

 

[rhd]

Hmm...It's been since high school since I used a 555 timer. Depending on cost and size issues I would use an Arduino like board connected to a relay.

I've been playing around with a Teensey 2.0 ($16) for a little while now and I don't think it would take much to get it to do what you want.

A 555 and the associated parts will cost about $1.00 for that circuit.
You don't need a $16.00 MCU for such a simple circuit..

@rhd...
you are on the right track... just test the circuit on a breadboard..


Jerry

Thanks for double-check. I thought I was on the right track.

Why wouldn't a 555 drive a relay? I've actually done that before (in my SLR time lapse controller).

 

[Trevor]

If I were you I'd drop an ATMega328p into the circuit to give some extra flexibility if you wanted to do some different output modes.

-Trevor

 

[Benm]

The output current is limited to 200 mA, and it has little or no protection against the voltage spike coming from a relay coil when it is switched off. I guess it could work when you run 12/15 volts supply voltage and connect a small relay, but i wouldn't trust it to work reliably.

 

[jcranmer]

For me it's about what's easiest not necessarily cheapest. I agree my example was over kill.

That's just me though. I can code quicker then I could figure out the correct circuit timing with a 555.

 

[lasersbee]

Thanks for double-check. I thought I was on the right track.

Why wouldn't a 555 drive a relay? I've actually done that before (in my SLR time lapse controller).

As mentioned above it is not a good idea to run a relay directly
by the output of the 555.

All you need is a 1K resistor between Pin 3 of the 555 and
the Base of a 2N3904 NPN transistor.

Connect the Emitter of the 2N3904 to ground and the
Collector to one side of your Relay coil. The other end of your
Relay coil goes to +VDC.

You will need a rectifier diode across the Relay coil with
the Cathode towards the +VDC.

Don't forget to connect pin 4 of the 555 to +VDC and bypass
pin 5 with a 0.1uF cap to ground.


Jerry

 

[rhd]

Query -
In the diagram posted in my original thread, is it crucial that the 0.1 uF cap be a non-polarized cap?

If that's not crucial (since all I have are polarized caps in 0.1uF), which way would the polarity go?

 

[Hemlock_Mike]

You might consider using a MOSFET instead of a relay and add a speaker to provide the "click".
I would point the + side of the 0.1 cap toward the IC since the trigger supplied - Gnd.
HMike

 

[rhd]

Worked!

The speaker would add too much bulk. This circuit can be TINY

 

[Hemlock_Mike]

There are speakers smaller than a relay ---
Consider a cell phone!
HMike