One transistor design

 

One Transistor Design

You might have already come across a host of these extremely basic one transistor FM transmitter circuits, however these may incorporate certain drawbacks as mentioned below:

• No substantial transmitting range.
• No enhanced sensitivity range
• Use 1.5V for operating which render limited capabilities.

Among the first in the line, which is probably the simplest is shown in the following circuit diagram.

Surprisingly it does not employ a MIC, rather the antenna coil itself performs a dual function of detecting sound vibrations and also transmitting it into the atmosphere.

The design is void of a frequency determining stage and thus does not come under tuned transmitter circuits (we’ll discuss about these later on in the article).

Circuit Operation

The following single transistor FM spy circuit may be understood as follows:

When switched ON, the capacitor 22n inhibits the transistor from switching until it gets charged. A soon as this happens the transistor switches ON via the 47k resistor forcing the pulse through the inductor which feeds back a negative pulse to the base of the transistor discharging the 22n capacitor.

This switches OFF the transistor until 22n yet again charges fully. The procedures take place rapidly generating a frequency across the coil which is transmitted as carrier waves through the connected antenna.

In the course if the coil is subjected to an external vibrational pulse, it’s forced to mount the above explained carrier waves in the air and could be received and retrieved over a standard FM radio positioned and tuned at the same frequency nearby.

The circuit may be expected to work at around 90MHz frequency band.