Ended up with this which seems to working. Look OK to you?

Not Bad. What voltages are you seeing on the FET outputs when on and off?
SG

 

Not Bad. What voltages are you seeing on the FET outputs when on and off?
SG

It's a bit hard to say as when its running the voltages fluctuate slightly as the LED switches on and off but for the control circuit there's no volt drop evident.

For the Tx though it does drop maybe 250mV (can't tell exactly as my meter sampling isn't quick enough) which is a concern. I'm using LP0701 FETs so maybe I need to look for one with a lower Ron than the 1.7 ohms noted on the datasheet?

I didn't check the opto volt drop but it is quoted as 4.8 so may have been worse....

 

Do you have any filter capacitors on the outputs of the FETs? I would suggest 100uf on both.
SG

 

Do you have any filter capacitors on the outputs of the FETs? I would suggest 100uf on both.
SG

What are they filtering?

 

What are they filtering?

They help smooth out the DC. It's general procedure to add them on the supplies even if powered by batteries. Do you have a scope?
SG

 

They help smooth out the DC. It's general procedure to add them on the supplies even if powered by batteries. Do you have a scope?
SG

Afraid not

 

My concern is the transmitter interfering with the control circuit. However from your observations using seperate batteries has eliminated that issue. If you are concerned about the voltage drop on the Tx supply then a lower Ron is viable. Only other issue which may not be a concern is when the power shuts off. With your latest design the power will not shut off "instantly" but rather drop off gradually as C5 starts charging.
SG

 

My concern is the transmitter interfering with the control circuit. However from your observations using seperate batteries has eliminated that issue. If you are concerned about the voltage drop on the Tx supply then a lower Ron is viable. Only other issue which may not be a concern is when the power shuts off. With your latest design the power will not shut off "instantly" but rather drop off gradually as C5 starts charging.
SG

Have a few SI2301CDS-T1-GE3 on order (Ron 0.1ohm) to try.

Once the 7242 is reset and the Tx has sent its transmission it doesn't seem to matter that the power doesn't cut cleanly. Actually, after the delay period it apparently drops instantaneously to 1V and then decays slowly to 0v over a few seconds (bit longer for the transmitter supply.

 

drops instantaneously to 1V and then decays slowly to 0v over a few seconds (bit longer for the transmitter supply.

OK that should be fine. Sure give those FETs a try.
SG