Circuit diagram:
Parts:
R1,R3,R4,R6______1K 1/4W Resistors R2,R5____________1M 1/4W Resistors C1,C6_________1000µF 16 or 25V Electrolytic Capacitors C2,C5___________47pF 50 or 63V Ceramic Capacitors C3,C4__________220nF 63V Polyester Capacitors Q1,Q4__________BC547 45V 100mA NPN Transistors Q2,Q3__________BD678 60V 4A PNP Darlington Transistors SW1,SW2_________SPDT Pushbuttons (or toggle/slide switches, see Notes) SP1,SP2________8-100 Ohm Loudspeakers (See Text and Notes) B1,B2_____________6V Batteries (See Text and Notes)
Comments:
The circuit is a modified version of an idea published by Ole Olmskov from Hoerning (Denmark) on WIreless World in 1979.
The original version of this outstandingly simple battery powered two-station two-wire intercom required two loudspeakers having an impedance comprised in the 25-120 Ohm range.
Unfortunately, such devices are almost impossible to locate nowadays, especially with diameters ranging from about 70 to 100mm. A loudspeaker diameter of this kind is indispensable to obtain good speech intelligibility and an acceptable efficiency.
Therefore, the circuit was modified in order to give a good performance even with common 8 Ohm speakers. The penalty is the relatively high current drawing when the circuit is operated. On the other hand, the intercom does not dissipate any standby power.
Circuit operation:
Two transistors and a handful of parts per station, are the only items required for this project.
Q1 and Q2 form a directly coupled, NPN-PNP configuration audio amplifier, allowing a high gain and a high current, low-impedance output.
As shown in the circuit diagram, pushing on SW1, the transmitting station hosts the actually operating amplifier (Q1, Q2 and related components), whereas the receiving station provides power to the transmitting station via its loudspeaker (SP2) and vice versa.
If the intercom is used for short periods and not too frequently, four 1.5V AA or C cells per station will last for a reasonable time. But you can also use rechargeable batteries, with the bonus that when the charger is connected, e.g. to B1, B2 will be remotely recharged through SP1, SW1, SW2 and SP2.
Notes:
- Please do not use supply voltages above 6V.
- Do not use speakers having an impedance of less than 8 Ohms and a diameter below 60 - 70mm.
- Some 16 Ohm loudspeakers suitable for this circuit should be still available on the market. If you will use them, the circuit performance will be greatly improved.
- At 6V supply and 8 Ohm load, the current drawn by the circuit is about 150mA. With a 16 Ohm load the current consumption will decrease to about 80mA.
- SPDT pushbutton switch types are best suited for SW1 and SW2, but you can also use toggle or slide switches in their place. But, please, try not to forget them in the "on" position...
- If the intercom is used for prolonged periods of time, fit a small, U-shaped heatsink to Q2 and Q3.
