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To switch a supply voltage to a device by means of a grounded switch, you can use a P channel MOSFET. If the switch is not pressed, then the P-MOSFET is not conducting. Something like this: simulate this circuit – Schematic created using CircuitLab


3

The circuit is fundamentally incorrect. The current produced by an (ideal) current source cannot be stopped by a series-connected switch; it must be diverted by a switch connected in parallel to the load (this is the so-called "current steering" technique). The reason for this is, as stated in the comments, that the current source will "try&...


2

Can this be done without oscillators or pulse counting stuff? Yes, but it's going to be difficult to get the precision you want. A circuit using several 555 timer ICs will do the job but you'll have to spend time trimming all your resistor and capacitor values to get the timing as you want. But "yes", it could be done. If this were my problem, I'...


1

If those are the parts on hand, this circuit should work. Resistor values are not critical and could be anything between 1K to 100K or so. Which transistor to use doesn't really matter much. The IRLZ44 has slightly lower RDSon so using that to switch the load is better. link


1

I knew I was missing something obvious, as soon as I posted the question it came to me. As far as I know, this circuit will work:


1

simulate this circuit – Schematic created using CircuitLab Figure 1. A 12 V relay provides a simple way of interfacing the old with the new. How it works: Pressing the button energises the relay. The relay contact can switch the 12 V supply as shown here or it can be wired in place of whatever would normally enable the new system. The advantage is ...


1

If low leakage and GOhm impedance is a prime concern, then I would not recommend CMOS switches, even if some are rated with quite low leakage currents. Moreover, for your application of switching caps in the pF range, the charge injection and switch capacitance are serious concern. If you really don't want to use relays, check Optically isolated MOS switches....


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