What is a LDR?
- When light is bright, resistance decreases
- When it is dark, resistance increases.
- Mostly used in automatic lighting, security systems and Automatic adjustment of shutter speed
What is a thermistor?
- Sensitivity about -4% / oC (-ve coefficient of temp.)
- When temperature rises, resistance decreases
- When temperature decreases, resistance increases,
- Good for temperature range from –50 to +300 degree celcius
- Inexpensive and stable
- Very sensitive to temperature changes because of large coefficient of temperature
- Mostly used in fire alarm systems, air-conditioning systems
How are LDRs used in automatic lighting circuits?
When light is bright
* resistance of the LDR is small
* V(B) is small
* transistor is in cut-off region
* bulb is OFF as no current flows through transistor
When it is dark
* Resistance in LDR increases
* VB is large
* Transistor is in active or saturation region
* current flows through bulb and turns it ON
How are thermistors used in Fire alarm circuits?
When a fire breaks out, temperature increases
* Thermistor resistance drops which causes VB to rise (Voltage Divider)
* Increase in VB results in VBE increasing until it reaches the 0.7V necessary to activate the transistor
* With transistor activated, current flows through the coil of the relay
* This closes the relay contact thereby triggering the electric bell
* Relays allow a low voltage circuit to control a high voltage circuit
What is the purpose of the free-wheeling diode in fire alarms?
- When transistor switches from ON to OFF, there is a collapse in the magnetic field around the coil
- This causes a back emf to be induced in the coil which must be discharged
- A diode is placed in reverse-biased across the coil, for energy to be safely discharged
- This protects the transistor from being damaged
What is a moisture sensor
- High resistance (open circuit) when dry
- Low resistance (closed circuit) when wet
- Mostly used in Monitoring of Soil Moisture condition or Automatic wind screen wiper or Automatic Clothes Hanging System
How does a moisture sensor circuit work?
Under dry conditions
* moisture sensor is a virtual open circuit
* No current flows into base of BJT
* Transistor is in cut-off region and motor is off
When raining
* Droplets of water fall onto the plate gap of sensor
* Base current now flows into the transistor
* Transistor becomes active (or saturated) and current flows through the relay coil
* Relay contacts change from “open” to “closed”
* Motor (connected to window closing mechanism or clothes-line retractor) will then be activated
