Controlling an RC car

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?

• Got a couple of NPNs or NMOSFETs handy? – Ignacio Vazquez-Abrams Nov 17 '13 at 23:17
• Is a schematic design possible? If I understood, when there is a signal, conductive area "R" gets grounded, otherwise not? – dempap Nov 18 '13 at 10:28
• 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. – Ignacio Vazquez-Abrams Nov 18 '13 at 10:36
• 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). – Ignacio Vazquez-Abrams Nov 18 '13 at 14:41
• 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)? – dempap Nov 19 '13 at 14:02