Astable Timer Circuit Diagram – Build Your Own!

Description

This timer circuit operates when the temperature drops below a predetermined level. An alternative version - Repeating Timer No.5 - functions while the temperature exceeds the specified level.

R7 allows for setting the desired temperature. The thermistor's value is not critical; the voltage on pin 1 is the important factor. Any thermistor value should function satisfactorily, though adjusting R7 may be necessary to achieve the desired temperature range.

Schematic Diagram

Setting The Timer

The circuit is essentially an Astable Oscillator. The output timing depends on the value of C1 and the rate at which it charges and discharges through the resistor network. The duration the relay remains energized is controlled by R1 and R2, while the duration it remains de-energized is controlled by R3 and R4. Fixed resistors establish minimum and maximum period lengths, and R2 and R3 set the maximum period lengths. With the component values indicated, both periods are adjustable from approximately 1 to 30 minutes.

Component values can be altered to suit specific requirements. If precise time periods are not needed and approximate values are sufficient, the potentiometers can be omitted, relying solely on R1 and R4 to set the times. Due to manufacturing tolerances, the exact length of the time periods achievable depends on the characteristics of the utilized components; a 4093 produces longer time periods than a 4011.

IMPORTANT

Do not use the onboard relay to switch mains voltage. The board's layout doesn’t offer sufficient isolation between the relay contacts and the low-voltage components. If you require to switch mains voltage, mount a suitably rated relay in a safe location - Away From The Board. I’ve used a SPCO/SPDT relay - but you can utilize a multi-pole relay if it aligns with your application.

Alternative Capacitor

During oscillator operation, the charge on C1 reverses polarity. Therefore, C1 must be non-polarized. However, a non-polarized 470uF capacitor can be simulated by connecting two 1000uF polarized capacitors back-to-back, as shown. Details on how and why this works are provided in the Detailed Circuit Description. Because non-polarized capacitors aren't readily available, the prototype was constructed using two polarized capacitors.

Veroboard Layout

Setting The Temperature

When R7 is turned fully to the left, it connects pin 1 to the positive line. Consequently, the timer will operate regardless of the sensor's temperature. In other words, there is no upper temperature limit. When R7 is turned fully to the right, the operating temperature is below 0ºC (32ºF).

This wide range provides coarse adjustment. Improved control can be achieved by incorporating a new fixed resistor (R8) in series with R7, and reducing the potentiometer value to, for example, 1k. R8 replaces the link from pin 14 to the R7 pole (it’s the link immediately to the right of the IC). Select a value for R8 that brings you close to the desired temperature, and use the reduced potentiometer value for fine adjustment.

Setting the operating temperature is best performed “in situ” - using trial-and-error. If the timer starts at too high a temperature, turn R7 a little to the right. If it starts at too low a temperature, turn R7 a little to the left.

The following readings provide an indicative understanding of the expected performance. These measurements were obtained from the CMOS 4011 prototype utilizing a cheap digital multimeter and a cheap thermometer. These are intended as guidance only, and identical readings are not expected.

My thermistor measured 4.45K at 25ºC (77ºF). My supply voltage measured 12v4, and the Pin 1 switching point was measured at 7v3. With R7 set to 8k, the timer operated at temperatures below approximately 5ºC (41ºF).

Temperature Sensor

The mass of the thermistor is small, and it will react quickly to changes in temperature. This makes it sensitive to thermal currents. A possible solution is to create an enclosed environment, where the thermistor is protected from momentary drafts, but can still respond to slower changes in the overall ambient temperature.

The Supply Voltage

The timer is designed for a 12-volt power supply. However, it will operate with a supply from 5 to 15 volts. You simply need to select a relay with a coil voltage suitable for your supply. I used a single-pole relay in the prototype – but you can use a multi-pole relay if it suits your application.