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555 Retriggering fault

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New Member
Hi all:
I'm building a simple 555-based timer circuit to control a relay. It is a monostable, negative edge triggered circuit, so that the relay is energized for up to two minutes but if the switch is held low past the two minutes, the 555 output falls, and the relay lets go. I'm using pin 4 (Reset) to stop timing if the switch is not held past the two minute critical time. I am using a protection diode, and another diode from the V+ to the coil, then a transistor to drain the coil to ground based on the 555 output into the base.

The circuit was working fine, I thought, so I wired up the main load, which is a peltier cooler for a bar refrigerator, and now it works sometimes. When it fails, the relay clicks for the briefest moment at the critical time (2 minutes, based on R1 = 470Ω and C1 = 220µF) but I guess it's getting retriggered. So there is noise in it somewhere, but I thought I did everything to suppress that, including the relay protection diodes, a small capacitor at the control (pin 5), and a smoothing capacitor (100uF) between V+ and ground.

Since I don't have an oscilloscope, I don't know for sure that the fault is happening at the trigger input, or somewhere else wierd.


Can you post the exact circuit you're using?


Well-Known Member
Most Helpful Member
Why is the door switch resetting the 555 when the door is open? Isn't that when you want the light on?

Also, what is the voltage and current of the light?

Also, I'm suspicious that the dc-dc converter doesn't have enough oomph to drive the light relay. Get rid of the diode in series with the relay coil; it does nothing except drop voltage you probably need to pull-in the relay coil. The snubber across the relay coil is a good idea.

I'm also suspicious that a presumably CMOS XOR gate operated off 5V could directly pull in a relay.
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New Member
Thanks, MikeMl:
The door switch (normally open connection) drops reset voltage when the door is closed (when the switch is pushed, NO & Common are connected), so the 555 output drops, and the light relay turns off.

Right now, I'm running a jumper to a solderless experimentor with a resistor and LED to simulate the light. When finished, it will be 120VAC and 0.3A.

The converter is 1A, and both relays are getting energized reliably: each draws about 90mA. I was also surprised that the XOR chip drove the relay, but it does get energized, so I figured, why amplify it?

Maybe I should use a diode from 555 out to XOR "B"?

I have been using the following site for a lot of assistance:
555 and 556 Timer Circuits
On that page, they said
"However, the 555 and 556 require an extra diode connected in series with the coil to ensure that a small 'glitch' cannot be fed back into the IC. Without this extra diode monostable circuits may re-trigger themselves as the coil is switched off!"
Although they are showing the coil driven directly by the 555 output.
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