July 31, 2021

# Tag: phone

## Telephone misuse indicator

When the phone line is in idle condition (on hook), the full voltage of 48V DC from the exchange appears across the line. When the phone is engaged by lifting the hand set, the voltage across the line falls down to around 12V. Thus, under idle condition (48V), VR1 is adjusted to give a positive biasing voltage to the base of Q3. As Q3 conducts Q1 gets into reverse bias and stops conduction. If the phone is engaged, Q3 stops conducting due to voltage drop across the lines, and subsequently, Q1 and Q2 starts oscillating and beeping is heard. Thus, this circuit can be used as an indicator for telephone tapping and misuse by an unauthorized person.

## Voltage Divider Rule

A Voltage divider consists of two resistances R1 and R2 connected in series across a supply Voltage Vs. The supply Voltage is divided up between the two resistances to give an output Voltage Vo which is the Voltage across R2. This depends on the value of R2 relative to R1:

If R2 is much smaller than R1, Vo is small (low, almost 0V)
(because most of the Voltage is across R1)
If R2 is about the same as R1, Vo is about half Vs
(because the Voltage is shared about equally between R1 and R2)
If R2 is much larger than R1, Vo is large (high, almost Vs)
(because most of the Voltage is across R2)