Figure 1 is a fan-made electronic cable for duplex intercom (intercom wired telephone can be called) circuit diagram. The machine has a call, two-way intercom, micro-power standby and other functions, for the public communication network coverage in the blind spot and there are special requirements for places to do private talk tools.
Circuit includes: microphone circuit, microphone amplifier, anti-side tone circuit, power amplifier, ringing circuit unit circuit.
Produced by two radio circuit in Figure 1, with two wires, connected in the correct polarity, to form one-wired phone system.
Working:
The two radio hook (switch s1 set two "hook" state), close the power switch S2, the system is in standby mode.
System, two walkie-talkies, no host, ext difference. When the system is in standby, the first pick up the calling party side, the other for the called party. Is agreed: element code plus "'" for the called party components, no "'" as the call side components. Special machines work, the battery pack GB, GB 'to the ringing circuitry; speaker BL, BL' connected to the VT4, VT4 'emitter circuit, due to resistance R9, R9' end of the tone control circuits TG, TG 'is set low, so the IC2, IC2' no ring signal output, BL, BL 'are not pronounced. C13, C13 'to prevent false triggering pulse burst interference ringing circuit.
When one party to talk with each other, you should pick up (the Sl set a "Call"), this time, lost their jobs caller ringing circuit voltage; the call circuit (the circuit in Figure 1 microphone, microphone amplifier, consumer side tone circuit, power amplifier circuit, the call light circuit, the call key) are energized to work VD2 call indicator light; speaker BL transferred to the output circuit IC1; press the call button SB caller positive voltage, through the connection line L1, R8 'to the called party ringing circuit TG' trigger signal, when the C13 'The potential is usually the trigger threshold, IC2' is triggered, OUT 'end of the output ring signal, the VT4' enlarge to promote the speaker BL 'a call tone. After the called party hears a call hook (the s1 'call to "talk") loses his job called party ringing circuit voltage; speaker BL' are diverted to IC1 'output circuit, the call tone to stop. At this point, the called party also received a call circuit voltage, the call indicator VD2 'light, the system to complete a call, the two sides can be no conversion interactive call, the simplex system, the two sides can only send / receive between conversion Responsive call. Call ends, both sides should be reliable hook (the s1, s1 're placed in "standby"), waiting for the next call to call. Hang up, the call light goes out, no one hang up is not reliable will affect the other calls.
When both sides are placed in the "call" when pressing the call key to any party, will not trigger the other ringing circuit, the transmission coupling capacitor C7, C7 'DC blocking effect, nor will it have any effect on normal conversation. Party only when the system is in "Call" and the other in a "standby" when the party in the call (calling party), press the call button will make the standby side (the called party) is ringing circuit is triggered, the BL 'a call tone.
Introduction to unit circuit
BM with electret microphone, the electro-acoustic performance, field-effect transistor to be provided on the built-in voltage. Figure 1, R1, RP1, C1 microphone power supply circuit components. For the filter capacitor C1; regulation RP1 can change the sensitivity of the microphone pickup.
VT1, VT2 pose a direct coupled, composite offset, the depth of negative feedback the microphone amplifier. This circuit, the operating point stability, good low frequency response, the circuit is simple and convenient debugging. R2 is the collector load resistance VT1, VT2 also provide the base for the bias; R3, R6 are VT1, VT2 series emitter current negative feedback resistor, R4 across the base of the VT1 and VT2 emitter, both the parallel negative feedback resistor, and to provide DC bias for the VT1. C4 is used to prevent the amplifier self-excitation, C3 can inhibit high-frequency noise. Provide access to low-frequency signal C5.
The two amplified audio signal directly coupled to the VT3, T constitutes the anti-side tone circuit. In the emitter circuit of VT3 with anti-side tone then the transformer T in the primary coil, C6 is a high frequency bypass capacitor. Radio circuit with low output impedance, high input impedance, and high signal to noise ratio, while more radical anti-side tone. "Side tone" refers to the duplex call, the side of the receiver (speaker or headphones), I heard his voice, the sound of the formation of acoustic feedback, it will interfere with normal calls. Thus in a telephone circuit, should minimize the side tone interference. Corresponding circuit is called a side tone cancellation circuit. Figure 2 is a side tone cancellation circuit of this equivalent circuit, the emitter-follower is equivalent to a low internal resistance of the signal source, set the signal source is a sine wave AC signal, the electromagnetic coupling, the secondary production in the same size T, phase Instead of the two sine wave signals, the two signal is applied to both ends of the load potentiometer RP2, RP2 regulating the dynamic side A, its sine wave output in the synthesis of A point is zero. This, added to the primary signal T from the A side can not enter into the side of the power amplifier circuit. But only to each other by the coupling capacitor C7 transfer. Similarly, other anti-side tone circuit by the signal can be sent by the A-side into the side of the power amplifier circuit.
Power amplifier IC1 is a low-power single-supply amplifier IC LM386, 9V power supply, it can give power 250mW 8O load. RP3 for the volume control potentiometer, access R7, C11 can change the output frequency characteristics. BL and the ringing circuit public speaker, by the switch s1-2 control.
This machine is the DC power supply, AC power in a region, should be replaced by AC / DC adapter. Microphone circuit, microphone amplifier, anti-side tone circuit consists of R11, VD1, C15, C16 constitute the regulator circuitry.
R10, VD2 composition of the call light circuit, "hang up" to be extinguished when the VD2. "Hook" should be light, the purpose is to visually display the working status of radio.
Component selection and commissioning
This machine adopts universal components: VT1 ~ VT4 as ß = 40-60 of the NPN low power transistor; resistance of various types can be optional 0.25W; capacitor C7 of the 600V AC voltage should be more than the rest of the large pressure at 16V on the line. Anti-side tone transformer T can be used in push-pull output transistor radio transformer, primary and secondary, in turn use, load impedance 8O, primary and secondary 2 × 3:1 ratio of the best, but also his wound; switch S1 for the 2 × 2's toggle switch, or special telephone hook switch.
Debugging, to take the collector current of VT1 0.6 ~ 1mA, VT2's Ic2 = 0.8 ~ 2mA, change R4 to adjust the overall gain of the amplifier, but the R4 will have general VT1 closed.
Anti-side tone circuit debugging a little bit more complex, the T of the original while adding the audio signal 0.5Vp-p, the amplifier volume to maximum, adjust the amplifier output RP2 smaller the better.