Showing posts with label telephone. Show all posts
Showing posts with label telephone. Show all posts
Saturday, March 23, 2013
Telephone Line Indicator
With the aid of an (old) moving coil instrument it is very little effort to make a simple voltmeter that, at a glance, indicates the status of a telephone line. Because the input impedance of this circuit is very high, there is no problem in having it permanently connected to the line, since it only draws a tiny amount of current. The schematic shows that the circuit consists of no more than a series resistor, a bridge rectifier and a moving coil meter. The value of the resistor depends on the sensitivity of the moving coil meter. In his prototypes, the author used old VU meters that require 250µA for full-scale deflection. A resistor value of 390 kΩ appeared to be optimal for these meters.
For a 100µA instrument, this resistor value will have to be increased to about 680 kΩ. The starting point, when selecting a resistor value is that when the telephone is not in use, the meter should deflect about 2/3rd of full scale. The amount of meter deflection indicates the three different states of the telephone line: 1. The deflection is very small: the line is in use (voltage 5 to 12 V). 2. The deflection is 2/3rd of full scale: the line is not in use (voltage typically 48 V). 3. Full-scale deflection: ring signal (60 to 90 V AC). Because the idle voltage and certainly the ring voltage are high enough to be dangerous, it is recommended that the circuit is constructed in such a way that it presents no hazard when touched.
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For a 100µA instrument, this resistor value will have to be increased to about 680 kΩ. The starting point, when selecting a resistor value is that when the telephone is not in use, the meter should deflect about 2/3rd of full scale. The amount of meter deflection indicates the three different states of the telephone line: 1. The deflection is very small: the line is in use (voltage 5 to 12 V). 2. The deflection is 2/3rd of full scale: the line is not in use (voltage typically 48 V). 3. Full-scale deflection: ring signal (60 to 90 V AC). Because the idle voltage and certainly the ring voltage are high enough to be dangerous, it is recommended that the circuit is constructed in such a way that it presents no hazard when touched.
Tuesday, March 19, 2013
Electronic Telephone Ringer
This circuit produces a ringing sound similar to that made by more recent telephones. It consists of three almost identical oscillators connected in a chain, each generating a squarewave signal. The frequency of each oscillator depends on the RC combination: R4 and C1 around IC1.A, R8 and C2 around IC1.B and R12 and C3 around IC3.C. The pairs of 100 kΩ resistors divide the asymmetric power supply voltage (between 5 V and 30 V) so that, in conjunction with the 100 kΩ feedback resistors (R3, R7 and R11) either one third or two thirds of the supply voltage will be present at the non-inverting inputs to the opamps. The voltage across the capacitor therefore oscillates in a triangle wave between these two values.
Electronic Telephone Ringer Circuit diagram
The first oscillator is free-running at a frequency of approximately 1/3 Hz. Only when its output is high, and D1 stops conducting, can the second oscillator run. The frequency of the second oscillator is about 13 Hz, and optional LED D3 flashes when it is running. When the output of the second oscillator is low, the third is allowed to run. The frequency of the third oscillator is around 1 kHz, and this is the tone that is produced. The second oscillator is not absolutely necessary: its function is just to add a little modulation to the 1 kHz tone. A piezo sounder is connected to the output of the third oscillator to convert the electrical signal into an acoustic one. The current consumption of the circuit is just under 1mA with a 5V power supply, rising to about 1.65mA with a supply voltage of 15 V.
Source: http://www.ecircuitslab.com/2012/07/electronic-telephone-ringer.html
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