How to Design Schematic Circuit of Over-Temperature Alarm

 This circuit will produce intermittent beeps by a piezoelectric transducer when

the temperature rises over a preadjusted value. You can use this circuit in

greenhouses, heaters, etc.The Sensor is a common silicon diode but an NTC

can also be used if you change some components'values.

Exchanging positions of R1 and Pl with Dl will cause the device to operate

as an under-temperature alarm.

The intermittent signal is generated by two oscillators. lGla (pins 1,2 and 3)

is an inverter that controls lCl b and lOl c (oscillators). lOl c (pins 8, 9 and 10)

is a very-low-frequency oscillator that provides the modulation rate for the

second oscillator. The second oscillator (pins 5,6 and 4) has its frequency

determined by R2 and C1.The produced audio tone can be altered by adjusting both C1 and R2 within a large range of values.

Power comes from a 3- to lS-volt power supply" For portable use a small 9-

volt battery is recommended. Current drain is very low, about 0.5 mA when

the tone is otf. When the tone is on, current drain rises to about 5 mA.

A schematic of the Over-Temperature Alarm is shown in Figurc 1. lOl, a

40ggB, has two gates operating as oscillators and two as butfer'inverters.

Components placement on a homemade printed-circuit board is shown in

Figure 2.You can made your prototype using a single-sided PC board.

Position of the polarized components, especially diode D1 (used as sensor),

should be observed. This sensor can be placed far from the device by using

common wiring. You must take some care to avoid humidity or water falling

onto the sensor, which will cause adversely atfecthe alarm operation.

BZ is a piezoelectric transducer (Radio Shack 27g-O7g or equivalent) or a

crystal earpiece.

Operation is adjusted by P1. Set this potentiometer to get sound at the desired temperature. To get a more precise adjustment you can replace the

common trimmer potentiometer by a multi-turn potentiometer.

Get the sensor between your fingers to see how it works: when the temperature rises some degrees the circuit will produce aserie of beeps. lsolate the sensor, and after some second sound emission will stop.

Parts List - Over-Temperature Alarm

lC1 - 4093 CMOS integrated circuit

D1 - 1N914 or 1N4148 general purpose silicon diode

BZ - Piezoelectric transducer or crystal earphone

(Radio Shack 273-073 or equivalent)

R1 - 100,000 ohm, 1l4W,5olo resistor

R2 - 33,000 ohm, 114W,5% resistor

R3 - 2,200,000 ohm, 114W,5% resistor

Pl - 10,000,000 ohm trimmer potentiomer

Cl - 0.047 uF metal film or ceramicapacitor

C2 - 0.47 uF metal film or ceramicapacitor

C3 - 100 uF, 16 WVDC electrolytic capacitor