One-shot (monostable) powering Op--Amp circuit

[Pha5e]

I am trying to design a circuit that incorporates a one-shot (monostable), which in turn powers an Op-Amp circuit for the monostable On period.

I have considered the 7555 and MC74HC4538 ICs for the monostable. It seems that the 74HC4538 IC consumes less current and works down to a lower voltage. This is necessary as the circuit will be powered from a battery (charged via a mini solar cell).

I would like to know how I can set the monostable to operate just once even if there are multiple triggers? I'm assuming that this should be straightforward with the 74HC4538. The monostable should reset only when the circuit is powered off or the IC reset.

Also, if I want the monostable to power an Op-Amp circuit, do I just connect the output to a FET (Gate) and place the Drain and Source of the FET inline with the VCC rail of the Op-Amp?

Thanks.

 

[barry]

First of all, you're going to need more than just a one-shot; you will also need some kind of latch to disable the one-shot after it's been triggered once. Also keep in mind that the HC4538 is, by definition, a RETRIGGERABLE one-shot. This means that if a subsequent trigger occurs before the time-out period expires, the time-out period will restart.

And yes, you can just put a fet inline with the opamp, but you need to address the polarity and voltages of the fet with respect to the rest of the system.

 

[Pha5e]

Also keep in mind that the HC4538 is, by definition, a RETRIGGERABLE one-shot.

Are there any monostable ICs that do not re-trigger? If not, what is the easiest way to design a low-current drain latch?

 

[barry]

No single-shot monostables that I'm aware of. If you just have a CMOS flip-flop (eg, 74HC74) clocked by the rising edge of the monostable output, and have the /Q output of that flop connected to the enable of the monostable, that should work. You'll have to ensure that the flip flop comes up in the 'reset' state (Q=0).

 

[Pha5e]

Would the 74VHC221A Dual Non-Retriggerable Monostable Multivibrator work:

https://www.fairchildsemi.com/ds/74/74VHC221A.pdf

 

[barry]

You said you want the device to ignore multiple triggers, so, no, the 221 won't solve the problem. Every time the device sees a trigger (when it's in its idle state), it will trigger.

 

[Pha5e]

And yes, you can just put a fet inline with the opamp, but you need to address the polarity and voltages of the fet with respect to the rest of the system.

Does the Gate voltage have to be greater than around 2V to switch a FET On?

 

[barry]

The short answer is: Yes. If you are willing to add a BJT you can control your power with lower voltage.

 

[Pha5e]

Re: One-shot (monostable) powering Op-Amp circuit

The short answer is: Yes. If you are willing to add a BJT you can control your power with lower voltage.

The supply voltage is likely to be 1.2V from a single AA rechargeable battery. Can you provide a link to some example circuits that use a BJT transistor to switch a MOSFET at a lower Gate voltage.

Thanks.

 

[barry]

If you've only got 1.2V, you are not going to be able to turn your mosfet on. Period. You'd better rethink this.

 

[Pha5e]

Is there any other way that I can power an Op-Amp when the output of the monostable timer goes high - given the low voltage constraint?

 

[barry]

You could use a PNP. Use the Q/ output (active-low output) to drive the base (through appropriate resistor). Emitter goes to your supply, collector to your load.

But I have to ask: if you've only got 1.2 volts available, what's powering your monostable???