Wednesday, October 1, 2014
Simple Temperature Indicator Circuit Using LEDs
A simple temperature indicator circuit was required for a gas fridge in a caravan to show when the pilot light had gone out.
The sensing element used was a thermistor, attached to the outlet which is warm when the pilot light is on, A rod~type thermistor was used for cheapness, with a. resistance of about 3k atg20* CW Two gates of the 7400 provide a Schmitt trigger with a low hysterysis (determined by the 18k feedback resistor) and the third gate inverts that output, When the pilot light is on, the input of lC1a is high,. lC1c output is logic O and LED2 (green) is on. lf the pilot light `fails, the temperature falls, all gates change state, LED2 goes off and LED1 (red) comes on. The temperature at which the changeover takes place is set by the 1k preset.
The sensing element used was a thermistor, attached to the outlet which is warm when the pilot light is on, A rod~type thermistor was used for cheapness, with a. resistance of about 3k atg20* CW Two gates of the 7400 provide a Schmitt trigger with a low hysterysis (determined by the 18k feedback resistor) and the third gate inverts that output, When the pilot light is on, the input of lC1a is high,. lC1c output is logic O and LED2 (green) is on. lf the pilot light `fails, the temperature falls, all gates change state, LED2 goes off and LED1 (red) comes on. The temperature at which the changeover takes place is set by the 1k preset.
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