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schematic

simulate this circuit – Schematic created using CircuitLab

Hi, I am trying to replace RF remote switches with transistors and want to control them using raspberry pi GPIO pins.

  1. Here as you can see, the grounds are different, so the circuit is not working. If I try to connect the raspberry pi ground to emitter of transistor it works fine. But my problem is, if i connect emitters of 2 transistor 2 same raspberry pi ground, it would short those 2 pins. I am not sure about the circuit on the remote, so can't short those 2 pins. Can I use another transistor to connect the ground to the emitter, for which base is connected to the same raspberry pi GPIO pin? What is the best way to do this? I know optoisolators are better way, but i don't have them presently. Please suggest if i can achieve using trasnsistors.
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  • \$\begingroup\$ The place where you have the raspberry pi ground tied is not DC common on the remote. You need to find the remote's true DC common for that connection. If you are lucky, it MIGHT be the "-" lead of the battery. \$\endgroup\$ – Tut May 22 '14 at 12:44
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Use an analogue switch like this: -

enter image description here

It uses a logic supply rail as low as 1.6 volts but can switch signals that may be connected to supplies that are -5.5V to +5.5V. On-resistance is less than 1 ohm too.

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While your schematic does not actually show your switches as having separate grounds, I'll take your word for it.

I'd suggest using photo-MOSFETs instead of bipolar transistors. See the TLP4227G datasheet for an example. These optocouplers use LEDs with a 1.1 volt forward voltage and a 3 mA current, so your 3-volt outputs should work OK.

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  • \$\begingroup\$ Thanks. I ordered optocouplers. Can you please suggest, if i can use another transistor to connect to ground? \$\endgroup\$ – venu May 22 '14 at 16:13
  • \$\begingroup\$ I'm sorry, I don't understand. Tie what to ground? \$\endgroup\$ – WhatRoughBeast May 22 '14 at 23:39
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You need to feed the transistors with some volts maybe from 3.3V of PI and you need to connect the emitter of both transistors with GND of Pi since you will put voltage in those transistors from Pi.

See the first picture, then you'll understand if you haven't already.

Schematics

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    \$\begingroup\$ Please write you instead of u. We don't use SMS speak on EE.SE . Also, what's with this bro ? Is user1772974 (the original poster) your relative? \$\endgroup\$ – Nick Alexeev Jul 27 '14 at 22:09
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    \$\begingroup\$ Yo watz up ma homey? ;-) \$\endgroup\$ – Jarrod Christman Jul 27 '14 at 23:30

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