555 timer with a relay

[crchisholm]

I am using a 555 timer to close a circuit that fires the shutter on a camera when the resistance drops across a sensor. The output of the 555 is closing a 5v relay that closes the circuit that fires the camera. When the camera isn't hooked up and I test with pull-up resister and an LED across the 9v and the ground to see if the relay is closing, everything works perfectly. When I attach the leads to from the camera (2.5v and 0v) to the relay, then the relay somehow causes the 555 to fire over and over. I suspect that the inductance of the coil in the relay is firing the 555 through the output line. I have the camera set to keep the shutter open as long as the relay is closed.

If I set the camera to close the shutter automatically after being triggered, and the pulse from the 555 to be of shorter duration than the time it take to open and close the shutter, it works fine. If, however, the shutter is open longer than the pulse, the 555 will fire again and again. This is only a problem in that I wanted to control the shutter open time with the variable resister (thus the pulse width) of the 555 circuit.

:? Is there a way to use transistors to isolate the and close the circuit of the camera. I have tried a few simple transistor circuits and the leakage from the transistors was enough to trigger the camera. Is there a completely leek proof transistor?

:? Is there a way to keep the 555 from firing from activity on its output (3) line?

As you can tell, I am not completely sure of what I am doing.

Charlie

 

[mattg2k4]

You should have a diode across the relay, in your schematic the arrow would be facing up. When you remove voltage from an inductive component, the magnetic field collapses and generates a fairly high voltage. The diode prevents that voltage from blowing up sensitive components it's connected to.

As for the transistor, did you have a pull up/down resistor (depending on if you're using npn or pnp)? I believe even when the the 555's output is low, it is still above 0 volts, so using a pull up or down resistor, and maybe a larger base resistor would solve the leakage you have. It could also be the transistors were duds, did you test them?

 

[kirby89]

I wonder if you couldn't simply replace the electro-mechanical relay w/ solid-state to cure these problems. Have noticed that a low quality mech relay I was using threw out alot of RFI & caused the self-oscillation you describe. Once I switched to a more expensive mech relay the RFI was reduced to a single click in the automotive radio & the feedback oscillation was gone.

 

[Roff]

If your input pulse on pin 2 is longer than 1.1*R*C (the timing components), then the output pulse will be as long as the input pulse. If pin 2 goes low and stays low, then the relay will remain on. If your camera retriggers with the remote shutter control pulled low, then you will experience the behavior you described. If the relay is actually cycling, then it may be feedback onto the supply. Do you have a capacitor from pin 8 to pin 1? Try 10uF in parallel with a 0.1uF ceramic. Install the diode across the relay coil as described by mattg2k4. And what is that LED and resistor doing in series with the coil? That won't work. Don't put anything in series with the coil.
I don't have a simple way for you to shorten the input pulse. The standard solution is to use an RC differentiator, but this requires a fast risetime pulse of at least 5v p-p. You can get the risetime and the amplitude you need by moving your 10k resistor and sensor to one input of a CMOS Schmitt trigger (CD4093), invert its output by going through another section of the CD4093, and connect that section's output to a 0.1uF cap whose other end goes to pin 2 on the 555. You will also need 100k from pin 2 to +9 volts. Connect ALL unused inputs of the CD4093 to +9V or 0V. Be sure to connect pin 14 to +9V, and pin 7 to 0V.
Using a transistor to trigger your camera requires that the 0 volt pin on the camera be tied to the 0 volt rail of your circuit. I think I would use an optoisolator if I wanted to use a solid state switch. Otherwise, noise picked up on the common 0 volt line could cause problems.