A simple busy indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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Figure 1 shows a diagram of a simple busy indicator. It is intended for an apartment where several parallel-connected SLTs are installed in the rooms, and allows you to determine by the glow of the LED that one of the devices is talking or the handset is not in place. Unlike circuits similar in purpose, published in [L1 p. 140] and [L20 p. 46], this one consumes less current from the line in standby mode with fewer parts.
The principle of operation of the indicator is based on the use of the property of the transistor VT1, when operating in microcurrent mode, to provide a large gain. And since it is turned on without negative feedback, a small change in collector voltage is required to go from saturation to off. When the circuit is operating in its initial state, when 60 V acts in the TL, the transistor will be in saturation, i.e. there will be no voltage on the VT2 gate. As soon as the tube is removed from the device, the voltage in the line drops to 6 ... 15 V, which leads to the blocking of the transistor VT1 and voltage is applied to the gate VT2 from the divider formed by resistors R1-R2 and R3-R4. Its level is sufficient to open VT2. .
Rice. 1. Scheme
Rice. 2. PCB
The circuit is configured with a construction resistor R4 (SPZ-19a) so that the HL1 LED indicator stops glowing at a voltage of more than 16 V. The indicator glow interval will be 6,4 ... 15 V, which corresponds to the off-hook. For mounting the elements, the topology of the printed circuit board is shown in Figure 2. It fits easily in a standard telephone jack or inside a SLT.
Publication: cxem.net
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