Temperature Switch | Electronic Project Circuits

Posts Tagged ‘Temperature Switch’

Temperature controlled Fan 12V dc

Suitable for computer cooling fans
LED motor speed indicator

Requested by some correspondents, this simple design allows an accurate speed control of 12V dc fan motors, proportional to temperature.
A n.t.c. Thermistor (R1) is used as temperature sensor, driving two directly coupled complementary transistors wired in a dc feedback circuit.
An optional circuitry was added to remotely monitor fan operation and to allow some sort of rough speed indication by means of the increasing brightness of a LED.

Parts:
R1______________15K @ 20°C n.t.c. Thermistor (See Notes)
R2_______________1K5 1/4W Resistor (See Notes)
R3_______________1K 1/4W Resistor
R4_____________270R 1/4W Resistor
R5______________22K 1/2W Trimmer Cermet or Carbon
R6_____________680R 1/4W Resistor (Optional, see Notes)
R7_____________470R 1/2W Trimmer Cermet or Carbon (Optional, see Notes)
C1_____________100µF 25V Electrolytic Capacitor
D1______________LED (Optional, any shape and color, see Notes)
Q1____________BC547 45V 100mA NPN Transistor
Q2____________BD140 80V 1.5A PNP Transistor
M1____________Fan Motor 12V 700mA max.

Please read more at the original source: http://www.redcircuits.com//Page109.htm
Thank you.

Be the first to comment - What do you think? Posted by admin - January 28, 2008 at 5:46 pm

Categories: Electronic Control, Motor controller Tags: Motor controller, Temperature Switch, Temperature-Control

Frost Alarm or Cold Activated Switch by IC 741

A simple thermistor triggered cold switch with adjustable threshold.

The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees Celsius. A suitable bead type thermistor can be found in the Maplin catalogue. The 100k pot allows this circuit to trigger over a wide range of temperatures.If using a different thermistor then the control should match the new thermistor at its highest resistance, or be higher in value. The op-amp in this circuit is the ubiquitous 741. It may be catalogued as LM741, CA741 etc, all types will work. In this circuit it is used as a comparator. The non-inverting input (pin 3) is biased to half the supply voltage. The non-inverting input is connected to the junction of the thermistor and potentiometer. The control is adjusted so that the circuit is on when the thermistor is at the required temperature range. Once the thermistor is outside the temperature range its resistance alters and the op-amp output changes from near full supply to around 1 or 2 volts dc. There is insufficient voltage to turn on the transistor and the relay will not energise.
A slight amount of hysteresis is provided by inclusion of the 270k resistor. This prevents rapid switching of the circuit when the temperature is near to the switching threshold.

Email: anc@mitedu.freeserve.co.uk
Source:: http://www.zen22142.zen.co.uk/Circuits/Switching/coldsw.htm