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I 'm planning on controlling an RC car via ΒeagleBoard-xM. Pistol transmitter uses ic TX2 encoder.

As image shows, right-left commands are implemented by a 3-part conductive area. Specifically, when area "R" gets grounded, a signal for right rotation is being transmitted. When area "L" gets grounded, a signal for left rotation is being transmitted.

The idea is BeagleBoard-xM to send these signals by two different GPIO ports. Presence of voltage in GPIO port, means signal to be transmitted. However, when I connected two wires for right rotation (one in conductive area "R" and the other to GNG) signal has been transmitted when I powered supply the circuit. This makes sense, as area "R" was already grounded.

The aim is a signal to be transmitted, when wire in conductive area "R" has voltage by BeagleBoard-xM. Is something like this possible?

enter image description here

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    \$\begingroup\$ Got a couple of NPNs or NMOSFETs handy? \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 17 '13 at 23:17
  • \$\begingroup\$ Is a schematic design possible? If I understood, when there is a signal, conductive area "R" gets grounded, otherwise not? \$\endgroup\$ – dempap Nov 18 '13 at 10:28
  • \$\begingroup\$ And N-type devices are used to pull connections to ground (see: "low-side driver"). Which N-type device you need depends on the characteristics of the signal and connection. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 18 '13 at 10:36
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    \$\begingroup\$ The collector goes to the "R" connection. And you need a resistor on the base since pulling the base high will result in putting Vcc and Vbe in parallel. Not a good thing. You'll get Ice(sat)(max), hfe(min), and Vbe(max) from the transistor datasheet and Vout(max) from the MCU datasheet, so you'll need to calculate Ib(min) and hence Rb(max). \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 18 '13 at 14:41
  • \$\begingroup\$ Just for confirmation: Ice(sat)(max) is the same as Ic (maximum rating)? As transistor voltage supply, I consider Vout(max) from MCU, which connects to the Base. Can I power supply Collector, by connecting it with pin 9 (VDD pin of the ic TX2)? \$\endgroup\$ – dempap Nov 19 '13 at 14:02
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Simple optocouplers will do the the trick (for instance MOCD217M, dual optocoupler, ~ $1 per unit). Basically, an optocoupler is a light-based relay and can be controlled from low power outputs, such as GPIOs. Using optocouplers will make sure that the emitter and the BeagleBoard electronics wont mess which each-other.

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