Wireless Projector Remote

A few years ago, we installed a projector in the church sanctuary. It is mounted to the front of an exposed truss, and the laptop that controls the video display is in the back of the sanctuary. The problem is that to turn the projector on or off, someone always needed to go to the front of the sanctuary and point the remote at the projector, which is not ideal.

This is my attempt at solving that problem. The idea is that instead of using an infrared signal from the back of the sanctuary, we'd have a key fob RF transmitter that would send a signal to a receiver. The battery-powered receiver would be placed somewhere at the front, pointed at the projector, and would translate the RF signal to the IR on/off remote commands that the projector understands.

Design features:

Project status

The RF transmitter and receiver circuits work to the point where I can get good carrier wave detection at about 30 feet.

The receiver, however, consumes way too much battery power, because the inductor I used for the boost regulator doesn't have a high enough saturation current. The bill of materials (below) reflects this change in the receiver component L2. I have not resized the land pattern on the board yet - it's still sized for an 0805 component.

The project has since been abandoned, because we now have a new projector with a remote powerful enough to bounce off of the front wall.

Bill of materials


B1 - Coin cell holder: CR2032   MPD BU2032SM-HD-G
C1 - Filter capacitor: 10µF (0805) Any appropriate
C2 - Antenna impedance matching: 6.8pF (0805) Any appropriate
C3 - Antenna impedance matching: 10pF (0805) Any appropriate
C4,C5 - Oscillator capacitors: 18pF (0805) Any appropriate
C6 - Filter capacitor: 0.1µF (0805) Any appropriate
D1 - Activity LED: 10mA typ., Red (1206) Lite-On LTST-C150KRKT
D2 - Power control circuit diode: Schottky, Vr>5V (SOD-323) ON Semiconductor MMDL101T1G
L1 - Antenna impedance matching: 470nH, SRF>315MHz (0805) Any appropriate
L2 - Antenna impedance matching: 150nH, SRF>315MHz (0805) Any appropriate
R1 - Current-limiting resistor for LED: 120Ω (0805) Any appropriate
R2,R3 - Power control pull-down resistors: 1MΩ (0805) Any appropriate
SW1,SW2 - Tactile switches: 7mm high, 160gf (6mm) E-Switch TL3301FF160QG
Q1 - Power control transistor: N-channel MOSFET, small signal (SOT23) NXP 2N7002E,215
U1 - Microcontroller: 8-bit PIC (SOIC-8) Microchip PIC12F615
U2 - Transmitter IC: 315MHz on-off keyed (SOT23-6) Micrel MICRF113
X1 - Oscillator crystal: 9.84375MHz (HC49/US) Abracon
N/A - Battery: 3V, 225mAh (CR2032) Panasonic CR2032
N/A - Keyfob enclosure: 2 buttons+LED window   Teko 11122.4


B1 - Battery terminals: Bent paper clips   N/A
C1 - RF data slicing capacitor: 0.033µF (0805) Any appropriate
C2 - Antenna impedance matching: 5.6pF (0805) Any appropriate
C3,C5 - Filter capacitors: 0.1µF (0805) Any appropriate
C4 - RF gain control capacitor: 1µF or 2.2µF (0805) Any appropriate
C6,C7 - Boost regulator capacitors: 22µF, tantalum (3216 metric) Any appropriate
D1 - IR LED: 20mA typ., 880nm (1206) TT/Optek OP250
L1 - Antenna impedance matching: 56nH, SRF>315MHz (0805) Any appropriate
L2 - Boost regulator inductor: 100µH, >250mA (1007) Taiyo Yuden CB2518T101K
R1 - Current-limiting resistor for LED: 220Ω (0805) Any appropriate
R2 - Noise squelch resistor: 10MΩ or 8.2MΩ (0805) Any appropriate
Q1 - Signal inverting transistor: N-channel MOSFET, small signal (SOT23) NXP 2N7002E,215
U1 - Microcontroller: 8-bit PIC (SOIC-8) Microchip PIC12F615
U2 - RF Receiver IC: 315MHz on-off keyed (SOIC-8) Micrel MICRF010
U3 - Boost regulator: 3V-to-5V boost (SOT89-5) NJR NUJ7216U50
X1 - Oscillator crystal: 9.7941MHz (HC49/US) Abracon
N/A - Enclosure: Battery compartment for two AAA cells   New Age Enclosures S-251605

Design files

Design details

RF antenna design

This being an RF application, I needed to pay special attention to sizing the various capacitors and inductors used in the RF blocks.

Transmitter antenna impedance matching

On the transmitter, I used the loop antenna from Micrel's example PCB layout. I used Micrel's specified matching network, with no modifications.

Receiver antenna impedance matching

On the receiver, I used Micrel's Heli-2 PCB antenna. For this one, I had to calculate a matching network. Impedance are given as complex numbers, the first component being resistance, and the second (the number prefixed by 'j') being reactance.

  • MICRF010 input impedance at 315MHz: 13.48 − j145 (from the MICRF010 datasheet)
  • Heli-2 antenna impedance at 315MHz: 11.2 − j216.1 (from Micrel app note 52)

This online tool calculated the following values for an LC network:

  • L: 0.05725µH
  • C: 5.5pF

Transmitter power supply

Since the transmitter is powered off of a coin cell battery, I didn't want it to be constantly drawing current for the microprocessor (although I could have used the microcontroller's ultra low-power sleep mode and wake-on-change feature for that). I tried to design a system where the ground plane is connected to the negative battery terminal through a MOSFET, which is activated by the buttons, and held active by an output pin on the microcontroller. The idea was that the microcontroller could decide how long to keep the power connected, then shut it off completely once the transmission completes.

Once I put the board together, I measured the off-state current and found that it is not zero. The problem, I surmise, is twofold. First, a small amount of current flows through the pull-down resistor for the MOSFET gate. Second, when the MOSFET is turned off, the ground plane is slowly pulled up toward the positive battery voltage, which also slowly raises the MOSFET gate signal wire, even with the pull-down resistor. The transistor finally reaches a balance point where it is partially on. It works well enough, but I would have been better off using the microcontroller's low-power sleep mode.

Work yet to be done

In order for this project to be workable, a number of things would still need to be done:

  • Hardware: Redesign the receiver with a better 3V-to-5V boost circuit.
  • Hardware: Redesign the transmitter to use the microcontroller's low-power sleep mode.
  • Firmware: Implement Manchester encoding in the transmitter (right now, it just sends alternating on/off signals)
  • Firmware: Implement Manchester decoding in the receiver
  • Firmware: Implement IR signal transmission in the receiver