Need to power-up/down auxiliary +5v Supply via Prop pin

Khrushchev

I have a prop circuit (see below) that uses a dual channel Vishay ILD213T optocoupler to interface a prop output and input to an external device requiring and returning a 5v signal. I have a separate (but not isolated) 5v supply for this side of the interface. I need to (occasionally) power the +5 side on and off via a prop pin. The 5v logic and load will draw less than 15mA. Could someone recommend a solution? I'm not experienced enough with FETs and Transistors to know the best and most economical way to do this. A circuit diagram would be most educational. Below is the current circuit as breadboarded. Pins 3 and 9 on J1 are prop pins.

kwinn

A single NPN transistor like a 2N3904 with a 1K base resistor could switch the low side (gnd) on and off. Add a PNP (2N3906) with another 1K base resistor to that if you must switch the high side (+5V) on and off. An optoisolator with a high CTR could also work for either high or low side switching.

max72

I had a similar problem, and Tracy Allen posted many useful suggestions and explanations here:
http://forums.parallax.com/showthread.php?142124-Low-power-guidelines
Massimo

Ariba

You can use the same circuit that you have between J1-3 and J2-3 to switch a 5V supply on and off. The 5V supply pin is then the output of the inverter gate. A 74LVC1G04 output can drive more then 32 mA at 5V according the datasheet.

Andy

Khrushchev

Thanks kwinn, Massimo, and Andy. All very interesting and educational. Andy, your solution was staring me right in the face and I missed it! I chose the LVC because I needed 5v logic with a little more than 8mA of output drive. Funny, when I went scrounging for parts for breadboarding, I found a few unused DM74LS04N's in my junkbox from the late `70s. They had sat there for over 30 years and never inverted one bit ... until now.

Rob