Month: November 2010
Up Counter
An up counter simply counts from 0 to 9. We can use this circuit to make digital object counter. This circuit can also be used to make a digital clock.
Continue readingDigital Object Counter using LDR and digital IC
555 timer configured in monostable mode is a simple automatic dark sensor circuit that gives output when light falling on LDR is blocked.
Continue readingBasic RC circuit
For the circuit to work as light operated musical bell, we need to connect the SW1- switch to N/C terminal of relay. LDR has to be kept in a dark case.
Continue readingFM transmitter and Dark sensor
A dark sensor can be converted to a simple FM transmitter using the following components. This FM transmitter is very sensitive and it has transmitting range of 30 meters.
Continue readingTelephone 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.
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