I have a toy that when a laser beam is broken triggers an alarm. I want to take the input to that alarm to trigger a camera - either from the alarm connection or tracing back to find where the alarm is powered. The device works from 2 AAA batteries.

The camera is a digital SLR that just needs a closed circuit to trigger the shutter and I just want it to trigger once - one closed then open pulse switch. I was thinking of using an SSR as these isolate the camera from the trigger source.

Any advice, especially schematics would be greatly appreciated.

Thanks in advance,

Rob.

 

A simple monostable will do the pulse.

555 Monostable

Use a phototransistor for side A switch.

A SSR (solid state relay) is a very high power optoisolator, used for switching line voltage. I suspect you just want an optoisolator, not a power switch.

 

Nice to get such a quick reply - thanks.

I apologise if this seems lazy but its been 25 years since I did any electronics and my camera is a prized possession but here goes the stupid questions:

So if I ground the control and threshold, put the signal to the trigger, pin 3 will pulse a closed circuit to ground? Reset connected to +Vcc.

Looking at the datasheet the 555 will work down to 3v but would you recommend that I use a separate power source (9v battery) to the unit's twin AAA 3v power supply?

Regards,

Rob.

 

Pin 3 goes high for a very short duration (a timed duration). You could use the pin 3 output to turn a transistor, which will short the collector to ground.

Pay attention how the LEDs turn on, the green LED turns on while the timer is on, the red LED is the normal idle condition (output is low).

Do you have clues to how the camera is triggered? No way for us to know without more info.

 

Thanks for your patience Bill. The camera is triggered by simply shorting a wire to ground - simple switch. This is what attracted me to a relay.

I am reading up on the 555 and I understand how the chip works. I'm not getting how this can provide me with a simple switch function unless I use a relay at the end of the circuit (which is fine but I don't want to over complicate things).

What I want to do is setup the laser beam toy. When the beam is broken an alarm sounds. I want to use the signal that triggers the alarm to make my camera take a picture. In this way I can get the water droplet pictures etc. As a later refinement I'll need to add a way of adjusting the trigger timing.

Cheers,

Rob.

 

Is there a voltage on the camera wire? Any guess as to how short a pulse will trigger camera? Is the V on the alarm buzzer DC or pulses?

 

Is there a voltage on the camera wire? Any guess as to how short a pulse will trigger camera? Is the V on the alarm buzzer DC or pulses?

 

Bernard,

Its a simple to ground switch - the usual trigger is just pushing a button that shorts the signal wire to ground, I believe it would only take the smallest of pulses to trigger the camera.

I haven't actually done any practical investigation on the circuitry yet as I want a firm idea of what to do and a good understanding of the implications - its a £1500 camera so I want to be well informed before I go poking about

My intention is to model the 555 circuit in Proteus but I'm having to learn that to get it done. Steep learning curve!

Regards,

Rob.

 

Sorry Bernard - forgot to say, the V on the buzzer is constant DC +3V.

 

I've done a similar thing with a slightly cheaper DSLR (Pentax K200D) and a ZVN2106 mosfet. I'm not sure if 3V is enough to turn it on, I was using 5V.
I think it's pretty safe, everything still works fine, but if anyone has any worries please mention them.

 

The attached bmp is what I've come up with - big thanks to HiViz web page and the people who have posted here. Just need to work out what's happening with the trigger in the laser receiver and I think I'm good.

Do you have a circuit for your design Mark?

Rob.

 

This something I drew up that might work. You need to make sure the positive connection goes to the collector, the negative to ground. In this configuration Q2 acts like a simple switch. Do not use a variable for R, because if it goes to 0Ω it will burn up the 555. If you put a pot for R use a second resistor to set a minimum value.

 

Bill,

Thanks so much for your help - this looks great. It should mean I don't even need the laser receiver.

I need to get some kit together, assemble a prototype and then I'll report back with what happens.

Regards,

Rob.

 

My circuit was really just a test. I ended up making an IR remote instead. The MOSFET was connected about the same as the bipolar transistor Q2 in Bill's circuit. The bipolar will switch with a lower voltage than a mosfet so is probably a better choice.

 

Agreed. Also, put a small tube in front of the phototransistor with the inside painted flat black. This will reduce the ambient light reaching the transistor Q1. The idea is the laser beam will make the trip, while everything else gets attenuated. It also means the transistor should be in dark environment (a really, really small box) too. Painted tape should work for this, and the tube doesn't need to be that long, I visualize 1" would do it.

If you do use a MOSFET you need a 12V power supply, the transistor I showed would work better in the lower voltage range (5V - 9V for the power supply).

Mark, I can't find a link or your image. Where is it?

 

I was just saying it was connected like in your circuit in post #12, but mine just had a switch instead of the timer circuit.

 

I won't guarantee mine will work, I think it will. It is possible the phototransistor will need additional gain after it to trigger the 555. I've just bought a batch of them for another project of mine (in the future) for a LED TX and RX using 555s.

Given you are using a laser confidence is high though.

 

Bill - I programmed your circuit into Proteus and it works great but I have to remove R2 to get it to work - am I being dumb somewhere?

 

R2 is absolutely necessary, that is a DC input, and it needs to be normally high. You can raise its value to over 1MΩ, but you need a resistor for the 555 to function properly.

Read the 555 Monostable article for a full explanation.

 

Thought I'd add an update as this thread has been so helpful - felt rude not to.

Bought a load of prototyping stuff - breadboard and components - and assembled a working prototype that turns an LED on and off. Bill's circuit was almost perfect - thanks again Bill. I have a cable to connect to the camera so I'll try that tomorrow. If the camera test works then I'll post the circuit.

Then I have to house it with mounting options so it can be setup to trigger pictures.

Cheers,

Rob.