Simple Signal Injector Circuit

Engineeering Projects

Here is a project of a Simple Signal Injector Circuit. Don’t think it’s just “another one”; see the advantages it offers over some other injectors:

  1. Commercial injectors working on 1.5V are rather large in size to be carried in a shirt pocket. You can carry this injector quite easily in a shirt pocket because the whole device can be assembled in a pencil cell holder.
  2. This circuit will give you a battery life of about two a single pencil cell.
  3. Most important of all, this injector will cost you at the most $0.50 (including battery), because the circuit contains just four components.

Description of Simple Signal Injector Circuit

As can be seen from the circuit diagram, there is no base bias for the transistor so that the transistor is ‘off’. Actually, as the specified transistor is AC128 (a germanium transistor), although it is ‘off’, a small leakage current flows in the circuit. This circuit ‘starts’ the circuit. When the battery is connected, leakage current flows through the high impedance primary of the driver transformer from point B to point A. This causes a momentary tiny current to flow in the low impedance secondary in opposite direction (E to F).

Capacitor C1 prevents the DC from the battery to be applied to the base but passes the momentary current induced in the secondary. As this current flows from the transistor’s emitter to the base, the base becomes a little negative with respect to the emitter for a short time. Due to this, there is a momentary rise in collector current, which gain induced a momentary current in secondary, and the cycle repeats.

The frequency depends on the L-C series resonant circuit formed by the transformer secondary and C1. Now, as the only current that is flowing is the leakage current (plus momentary increase), the total current is quite small (Standing current 25 uA and 50 uA when using the injector). There is no need for an on/off switch, and the battery can be used for about two years in continuously ‘on’ condition economical, isn’t it?

PARTS LIST OF SIGNAL INJECTORS CIRCUIT

Capacitors
C1, C2 = 10 kpf (Ceramic Disc)
Semiconductors
T1 = AC128 (germanium transistor) you can also use 2N3906 (silicon transistor)
Miscellaneous
X1 = Miniature Driver Transformer (This type used in Japanese pocket radios)

 

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