Re: Mictester's CMOS Relay Bypass Module

Posted by Freppo on
URL: http://guitar-fx-layouts.238.s1.nabble.com/Mictester-s-CMOS-Relay-Bypass-Module-tp19021p45381.html

There's a number of issues with that schematic, so I wouldn't recommend using it.

First, like purity mentions. The unused inverter outputs should not be connected to ground. Since it's an inverter, if the input is low (at ground) it will try to pull the outputs high, so it will basically cause a short between v+ and gnd (or very close to a short, which will draw alot of current) and it can damage the chip.

Second big issue is that it's missing a flyback diode between the relay positive and negative. It needs a diode (1N4148) with the cathode pointing at the positive side, to protect the other components from the discharge when the relay magnetic field collapses.

It's also way more complicated than it needs to be. You can also remove the whole transistor section which is unnessecery. A CD40106 inverter can sink enough current for most small relays. So a better way would be to keep the relay + side to v+ at all times and toggle the ground side insted directly from an inverter.

So keep the IC1A / IC1B section (this is the flip flop toggle) but remove C1 (not needed) and remove R1 (put a link there insted). Then connect the output of IC1B directly to the relay negative side. That's really all that you need. Don't forget the flyback diode, and you could use another inverter to have the LED isolated from the relay.

Cheers / Fredrik