555 timer issue

[skaskas]

Hello there, an Electronics newbie here!

I'm trying to delay the latching of a relay switch. Attached is the schematic that I'm having the issue with.The circuit is incomplete as I've hit a stumbling block.

Essentially, when the circuit is powered on, I plan to reset the latch in case it's closed by having a connection between the relays reset pin and positive voltage. Regardless of that, the first problem is that I want to start a delay timer when I press SW1. At the moment, this causes the 555 timers output to go immediately high for a few seconds, then it flips back low. I want to latch the relay when the timer is done, not when it starts.

The second problem is that when I press SW1, the relay latches but the timers output doesn't revert to low. It stays high. Basically, C3 does not discharge. It fills up and stays there. When I disconnect the connection between the 555 output and the relay, the timer operates as you would expect (C3 charges and discharges).

I replaced the relay with an LED (connected to a 1k resistor to ground) and the timer worked fine. The main difference I found between the relay and the LED was in the amount of current each drew when the 555 output was high. The relay drew 20mA while the LED drew 2mA so I think that's part of the problem. BTW, the relay is a DS2E-SL2-DC5V relay.

I'm sure that what I'm trying to do is much simpler than this, any light would be much appreciated.

switch.JPG (76.54 KiB) Viewed 7258 times

Cheers,
Saki

[pebe]

If you want the relay to close after the pulse then you should connect the relay between output and V+ supply. But that means that the relay would also be closed before you triggered the timer. If you only want the relay on after the pulse then you need a second 555 triggered as the pulse of the first one ends.

Your second problem is caused by the high back-EMF spike from the relay as current in its coil is cut off. The output pin goes highly negative and as it's next to the trigger pin, it generates a negative pulse on the that pin and triggers the timer again.
The cure is to fit a 1N4148 diode across the coil with its anode to ground and its cathode to the output pin. If you connect the relay coil between output and V+, then connect cathode to V+ and anode to output pin. In other words wire the diode so it does not conduct when the relay coil is energized.

Edit, 25 July:
Incidentally, you have shown C3 with wrong polarity.

[skaskas]

Thank you very much for the reply pebe. Much appreciated.

Saki

[khushboo bansal]

hello i want to ask you that can i use 555 timer circuit interface with micro controller circuit. i want to generate 40 khz signal and display it on micro controller. thank you

[pebe]

Yes, you can interface a 555 with a micro provided they both share a common supply voltage.

BTW. Yours is a new topic so you should have started a new thread.