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Basic Electronics » Reset switch for circuit

June 25, 2009
by sector0
sector0's Avatar

Fun problem :)

I have a circuit that requires a power reset to come from the circuit logic. Basically, I have power running to an IC that latches an output when an IR signal is received. The IC will only reset the output if the power is lost or if it receives another IR signal. What I want to do is receive the IR and then trigger a 555 timer that sends a pulse to reset the whole circuit after n amount of seconds.

Right now, I have the circuit working so that when the IR is received the 555 timer is powered and runs a cycle. However, what I need is a way to cut the power to the circuit near the voltage regulator. Is there a type of reverse power transistor switch so that current flows when gate is low but stops when gate is high?

The best thing I can come up with is a XOR near the regulator that has one pin on VCC and the other to the output of the 555 timer. Then when the pulse is received the XOR will go low since both inputs are high and reset the power causing the 555 timer to go low again, thereby restoring power (I guess more of a brown out type condition as I would not know how far voltage would go before the circuit would reset). The problem with this is it means I have to put more components in that I don't really want. I am hoping for a better idea ...

June 26, 2009
by Starwarslegokid
Starwarslegokid's Avatar

Hi sector0,

I believe a P-type mosfet is what your looking for, When the gate is low the mosfet is on. With a N-Type you have to make the gate high to turn the mosfet on. I think this is right, i would read further on the P-type mosfett B-)

Another idea is a normally closed relay, and then when power is applied the relay will open, cutting the power to your chip.

hope this helps B-)

Scott

June 27, 2009
by sector0
sector0's Avatar

Whoo hoo.. I had a 5vdc SPDT relay sitting around for something else and tried it out. That worked out beautifully. Basically the IR chip sends a high (when triggered) to a N-mosfet which turns on the timer. I have the relay coil hooked to the low side of timer and the relay has the IR chip ground on the N/C side with the relay going ground. As soon as the timer gets power from the mosfet it goes high which doesn't trigger the relay as it is connected low but when the timer goes low the relay trips (going to N/O) causing the IR chip to drop power and the whole system resets. Couldn't believe it actually worked on the first go around.

Now if only I knew it was safe to do it this way :)

I did read up on the P-Type MOSFET, and was not completely confident it is going to work. I don't have any laying around and I could not understand the workings from the datasheets. It would be nice to replace the relay with one of those...

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