Electronics > Projects, Designs, and Technical Stuff
Permanent Latch Circuit
(1/2) > >>
Pentoad:
Hello all,

This circuit here switches on a relay when a trigger voltage is applied and keeps the relay on permanently until power is reset.

What I need though is a system where the relay switches on when the circuit is powered up and switches off permanently when a trigger voltage is applied and stays off until power is reset. How would I change the circuit to allow this?

jbb:
Use the normally closed relay contact?
Pentoad:
That can't work for my application as it would allow the load to run even if there is no power going to the circuit or the circuit suffers some kind of failure.
Prehistoricman:
Here's something I just threw together in simulation.

Here's what the circuit does, in chronologic order:
This generator on the left is set up to deliver a power-up type slope over 5ms (to make sure the system performs with a realistic turn-on).
R1 initially turns T1 on, so that the relay turns on at power-up.
C1 makes sure that T2 doesn't turn on before T1 turns on
SW1 is set up to close after 10ms to simulate some external input going high
R3 limits the current into the base of T2 (this resistor must be smaller than R2)
T2 pulls the base of T1 low to turn off the relay
SW1 opens at 15ms to simulate the external signal turning off
R2 provides feedback to keep T2 turned on (the current flows through the relay coil)

I've attached a zip of the TINA-TI (that's the simulator I use) schematic, if you want to play with it.

Improvements that should be considered:
A resistor to discharge C1
Diode on the input, so that a low state cannot be asserted by the external signal
Zero999:
The circuit posted above is an example of a bistable multivibrator.
https://en.wikipedia.org/wiki/Bistable_circuit

C1 should discharge via the transistor's base when the power is remove, so it shouldn't be an issue.

The circuit can be made less sensitive to the input being forced low, by increasing the value of R3.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod