Relay control seems fine but goes astable when circuit is added

[mijenak]

I am trying to build something to trigger my camera when an infrared beam is broken. I used two kits

infrared beam kit
https://www.kemo-electronic.de/pdf/b216/b216.pdf
relay kit
https://www.kemo-electronic.de/pdf/b197/b197.pdf

I have connected the beam break kit as the control to the relay kit as shown. When I test the beam break, it seems to work. When I interrupt the beam I see the LED on the beam break kit switch on and hear the relay switch state. When I stop interrupting the beam, the LED goes out and the relay switches back.

When I connected a second circuit to the relay s and p (I used a simple 3v battery pack and torch bulb as a test), the LED on the control circuit and the relay both switch on and off rapidly. What could be causing this and how do I fix it? One thought is that the relay is designed to switch a higher voltage and current and I am using it for a low voltage and low current application.

 

[mijenak]

Anyone? I have this stuff all spread over the dining room table and my missus is starting to get narky!

 

[eblc1388]

When I connected a second circuit to the relay s and p (I used a simple 3v battery pack and torch bulb as a test), the LED on the control circuit and the relay both switch on and off rapidly. What could be causing this and how do I fix it?

Positive feedback. Ever use a microphone and heard feedback echo from the amplifier?

The light of your bulb shone on the photo-transistor of the "beam break unit" and the photo-transistor thinks the light beam is back.

Moves the light bulb away from the beam break unit and the astable oscillation will stop.

One thought is that the relay is designed to switch a higher voltage and current and I am using it for a low voltage and low current application.

No.

 

[mijenak]

Thanks for the reply. I will double check but I don't think that's the answer. The bulb was not that close to the photo transister and I seem to remember that the bulb didn't light up when the oscillation was happening. Presumably not enough continuous power getting to the bulb.

 

[eblc1388]

Thanks for the reply. I will double check but I don't think that's the answer. The bulb was not that close to the photo transister and I seem to remember that the bulb didn't light up when the oscillation was happening. Presumably not enough continuous power getting to the bulb.

Are you using a regulated 12V power supply as required by the operating instruction of the "beam break unit"?

If not, the relay during operation might take sufficient current and brought down the voltage of your power supply. This usually happens if an un-regulated power supply is being used and because the detection circuit takes so little current, any un-regulated power supply like wall-mart would give out a higher voltage when relay is not operating.

 

[Willbe]

You may need a single point ground, if heavy circuit current is modulating the voltage across sensitive circuit input wiring.
I saw one in a hifi set; this two inch long busbar had two dozen wires soldered to it, from all over the set.

 

[mijenak]

Are you using a regulated 12V power supply

No! I think you may be on to something there. I am using a small 8xAA battery pack because this (my application) is intended for outdoor use whereas I get the feeling the kits are generally intended for automotive use. I will retest with a good quality 12v supply but it's weird that it only goes astable when I connect the other circuit. I don't see the difference between the relay being closed but with no current flowing and with current flowing.

You may need a single point ground, if heavy circuit current is modulating the voltage

This is where I'm a bit confused. The two circuits have a separate power supply and are only connected at the relay because I want to isolate my expensive camera electronics from my dodgy home soldering efforts. The camera shutter release circuit is only something like 5.4 volts DC. So firstly it's not a heavy circuit that I'm trying to close with the relay. If I did connect the ground of the two circuits, I would need to be sure that I am not putting 12v into the camera! But I guess I can measure this with a test circuit.

 

[eblc1388]

I will retest with a good quality 12v supply but it's weird that it only goes astable when I connect the other circuit. I don't see the difference between the relay being closed but with no current flowing and with current flowing.

As the load is driven via isolated relay contacts, it has absolutely nothing to do with the load current or common ground as the two systems are isolated to each other.

Relay coil needs current to operate and often this current alone is sufficient to drop the voltage of a battery pack if the cells are not all new; a common cause as people just put together a battery pack from various 1.5V batteries found in hurry.

This is where I'm a bit confused. The two circuits have a separate power supply and are only connected at the relay

You're right on separate power supplies. Someone else is confused.

 

[mijenak]

Thanks for your replies. I will re-test over the weekend and let you all know if the power supply is the issue.

 

[mijenak]

I decided to test with 8 brand new AA batteries of a reputable brand and this seems to have solved the issue. The circuit is now stable.

Thanks all for your help. Also added a shielded cable to the photo transistor and this has improved the reliability further.

I have a new issue which I will search the threads for and start a new one if I can't find a fix.