Solar Powered FM Bug

(C) G. Forrest Cook 2010

June 8, 2010

Introduction

There are many miniature FM transmitter (bug) circuits online, this one is unique in that it runs completely on solar power. No battery is required. As long as the sun is shining on the PV panel, the circuit will transmit. The bug is useful as a "remote ear", and can be used for anything from listening birds to surveillance work. The mic preamp and oscillator circuits were borrowed from a common circuit found around the Internet, a regulated solar power supply and an RF amp that extends the range and improves frequency stability were added.

Theory

The solar power supply consists of a small 18V PV panel which charges a 1000uF electrolytic capacitor. The capacitor keeps the circuit running during brief interruptions of light, such as a bird flying over the PV panel. The 18V is regulated down to 9V with the 78L09 regulator IC to provide a steady 9V supply for the rest of the circuitry. The PV panel used only allows the circuit to work when direct sunlight is shining on the panel. A higher current panel such as a common 3W VW battery charger panel would allow the circuit to work in cloudy conditions.

The Electret microphone is biased with a 33K resistor, the resistor value can be changed to vary the amount of modulation. The microphone signal is amplified by a 2N3904 audio amplifier. This signal is sent to the 2N2222A oscillator stage where it changes the oscillator's frequency (FM). The oscillator's operating frequency is set by L1, the 6pF capacitor and the 5-20pF variable capacitor. With L1 wound as specified on the schematic, the circuit will operate near the low end (88Mhz) of the FM broadcast band.

The output of the oscillator circuit is taken from a tap on the oscillator coil L1 and fed to the RF amplifier 2N2222A transistor. The output of the RF amp is run through a low pass PI filter to remove unwanted RF harmonics before the signal is sent to the antenna.

Specifications

Construction

The prototype circuit was built using the "dead bug" construction method, grounded parts were soldered to a blank copper circuit board and connections were soldered between the parts as required. A perforated prototype circuit board could also be used.

It important to mount the oscillator components solidly so that they don't move around and cause unwanted frequency shift. The component leads for all of the RF wiring should be kept short. The coils were wound on a #2 Philips screwdriver shaft and stretched out a bit. To improve the circuit's frequency stability, wind the oscillator coil on a 1/4" plastic form, heat in an oven at low temperature for a while to anneal the metal, then apply "Q dope" to the coil.

The prototype circuit shown in the top photo was laid out as the circuit was designed. It could be neater, but it functioned correctly. A second-generation version of the circuit was built using a home-made printed circuit board, this is shown in the second photo. Artwork for the PCB is available at the end of this page.

Antennas

This circuit will work with a variety of antennas. An adequate short-range antenna can be as simple as a 1' to 2' wire connected directly to the circuit. For longer-range operation, a resonant antenna such as a dipole or a vertical antenna can be used. A resonant diple antenna for 88Mhz can be made with two 2.8 foot pieces of wire fed in the middle. the PV panel and wiring should be kept away from the antenna, or in the case of a short whip antenna, the PV wiring can be run in the opposite direction as the antenna to act as the other half of a dipole.

Alignment

The circuit can be aligned in the laboratory by putting 12V to 18V DC across the PV panel to power the regulator. Tune your receiver to a blank spot on the lower end of the FM band and adjust the frequency calibration trimmer until you hear the microphone signal. Turn the trimmer very slowly, alignment takes a light touch. Don't turn the receiver volume up too much or you will get audio feedback. A frequency counter may be useful for setting the output frequency. It may be necessary to retune the frequency a bit after the circuit has warmed up in the sun.

The output capacitor should be tuned for the maximum transmitted signal, this setting varies with different antennas. The best way to do this is to connect the antenna to the transmitter and monitor the signal with an oscilloscope (100 Mhz bandwidth) connected to a nearby antenna. Adjust the control for the highest signal. If you have a receiver with a signal strength indicator, that can also be used for monitoring the transmitter's output level. Adjustment of the output capacitor will pull the oscillator frequency a bit, it will be necessary to alternate between oscillator and output adjustments to fully align the circuit.

Use

Place the PV panel in the sun and tune your receiver to the bug's signal, listen to the world outdoors. An analog receiver is best for picking up the signal since, unlike a digital receiver, it can be fine tuned to track the signal. I use a 1970s vintage Pioneer receiver to good effect. Once the bug's temperature has stabilized, its frequency should not drift very much.

Parts

Resources

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