Relay triggering when power initially applied to circuit. - Page 1
Forum section: electronics / beginners

author: craftyjam
author_rank: Newbie
timestamp: April 16, 2022, 07:14:11 pm
content: Hello, I thought I knew something about electronics until I didn't.I have a super simple circuit, 555 astable multivibrator with the output going to the base of a NPN transistor (with a 1k base resistor) that energizes a 5v relay connected between the collector and 5v rail. Away from computer, but the schematic is just the basic 555 oscillator, with a momentary switch that attaches pin 1 to GND to start the oscillating.The circuit works fine, but when applying 5v to the power rails, the 5v relay seems to activate, even though the 555 pin 1 is disconnected. I imagine there is a voltage present at the base of the transistor, but I do not know how to remedy it.Any help would be appreciated, thanks.

author: jonovid
author_rank: Super Contributor
timestamp: April 16, 2022, 08:35:55 pm
content: state your objective? are you making a buzzer circuit?post an image. someone with more experience then I may help you.

author: MikeK
author_rank: Super Contributor
timestamp: April 16, 2022, 08:53:02 pm
content: Why are you switching the ground pin instead of the power pin? And I hope you have a protection diode across the relay coil so that you don't destroy the transistor.

author: craftyjam
author_rank: Newbie
timestamp: April 16, 2022, 09:06:33 pm
content: Quote from: jonovid on April 16, 2022, 08:35:55 pmThe objective of the circuit is an antique doorbell ringer. I set the frequency on the 555 to be about 180bpm, so I can simply press the push button and ring the bell as many times as I want in regular intervals. Makes a good dinner bell.Quote from: MikeK on April 16, 2022, 08:53:02 pmYes I have a emf protection diode across the relay coil. There is no particular reason I chose to switch the ground instead of the power rails, though, I suppose that could be causing the miss firing at startup.

author: ledtester
author_rank: Super Contributor
timestamp: April 16, 2022, 09:12:10 pm
content: Quote from: craftyjam on April 16, 2022, 07:14:11 pmDoes the switch only cut power to the 555 chip? Usually these circuits assume the external timing capacitor is initially at 0 volts.While you're at it, you might as well add a power-on reset circuit to the 555 so that it doesn't start up in a random state:https://electronics.stackexchange.com/questions/332279/how-do-i-reset-ne555-output-to-low-when-powered-onEven if it's a super simple circuit showing us a schematic would be very helpful.

author: craftyjam
author_rank: Newbie
timestamp: April 16, 2022, 09:22:09 pm
content: Quote from: ledtester on April 16, 2022, 09:12:10 pmThank you for the suggestions.Yeah, the switch only cuts power to the 555, it would probably be better to switch the whole 5v rale. I didn't know about the pull resistor and capacitor to set the low state. I'll give that a try.Sorry about no schematic, I can post it tomorrow.

author: Romualds
author_rank: Contributor
timestamp: April 17, 2022, 05:40:51 am
content: Yes please, schematic will help.Sometimes (or often) there are unexpected behavior of circuits during power on (power off) cycles. This is the same case you observe. Having the diagram should help understanding what kind of modification needed to fight the unneeded relay triggering.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 17, 2022, 02:10:53 pm
content: put an RC network on the reset pin to hold the 555 for a few mS.R to Gnd cap to Vcc.

author: jonovid
author_rank: Super Contributor
timestamp: April 17, 2022, 11:23:43 pm
content: if your running cable? a door bell, anything over 6 feet add induction or voltage drop to your circuit design.from my own experience the 555 likes a positive supply voltage of 6 to 12. particularly when driving a load.5 volts is the bare minimum with no room to spare.