CIRCUITIDEAS

E L E C T R O N I C S F O R Y O U • N O V E M B E R 2 0 0 5 • 9 9W W W . E F Y M A G . C O M

CMYK

A t the heart of this heat-sensi-tive switch is IC LM35 (IC1),which is a linear temperature

sensor and linear temperature-to-volt-

age converter circuit.The converter provides accurately

linear and directly proportional out-put signal in millivolts over the tem-perature range of 0°C to 155°C. It de-velops an output voltage of 10 mVper degree centigrade change in theambient temperature. Therefore theoutput voltage varies from 0 mV at0°C to 1V at 100°C and any voltagemeasurement circuit connected acrossthe output pins can read the tempera-ture directly.

The input and ground pins of thisheat-to-voltage converter IC are con-nected across the regulated power sup-ply rails and decoupled by R1 and C1.Its temperature-tracking output is ap-plied to the non-inverting input (pin3) of the comparator built around IC2.The inverting input (pin 2) of IC2 isconnected across the positive supply

rails via a voltage divider networkformed by potmeter VR1.

Since the wiper of potmeter VR1 isconnected to the inverting input of IC2,the voltage presented to this pin is lin-early variable. This voltage is used asthe reference level for the comparatoragainst the output supplied by IC1.

So if the non-inverting input ofIC2 receives a voltage lower than theset level, its output goes low (approxi-mately 650 mV). This low level is ap-plied to the input of the load-relaydriver comprising npn transistors T1and T2. The low level presented atthe base of transistor T1 keeps it non-conductive. Since T2 receives the for-ward bias voltage via the emitter ofT1, it is also kept non-conductive.Hence, relay RL1 is in de-energisedstate, keeping mains supply to theload ‘off’ as long as the temperatureat the sensor is low.

Conversely, if the non-inverting in-put receives a voltage higher than theset level, its output goes high (approxi-mately 2200 mV) and the load isturned ‘on.’ This happens when IC1 isat a higher temperature and its outputvoltage is also higher than the set level

at the inverting input of IC2. So theload is turned on as soon as the ambi-ent temperature rises above the setlevel. Capacitor C3 at this pin helpsiron out any ripple that passes throughthe positive supply rail to avoid er-rors in the circuit operation.

By adjusting potmeter VR1 and

thereby varying the reference voltagelevel at the inverting input pin of IC1,the temperature threshold at whichenergisation of the relay is requiredcan be set. As this setting islinear, the knob of potmeter VR1 canbe provided with a linear dialcaliberated in degrees centigrade.Therefore any temperature level canbe selected and constantly monitoredfor external actions like turning on aroom heater in winter or a room coolerin summer. The circuit can also beused to activate emergency fire extin-guishers, if positioned at the probablefire accident site.

The circuit can be modified to op-erate any electrical appliance. In thatcase, relay RL1 must be a heavy-dutytype with appropriately rated contactsto match the power demands of theload to be operated.

M.K. CHANDRAMOULEESWARAN ANDMISS KALAI PRIYA

HEAT-SENSITIVE SWITCHS.C. DW

IVEDI