Practical Electronics/Astables/Double Astable

This is a way of making a much more distictive signal form a ordinary astable signals. It needs at least two astables, with one set to a significantly higher frequency than the other. If the output of the astable with the longer period (lower freqency) is connected to the Pin 4 reset pin of the second astable, the following result is obtained:



The slave astable will not actually give the second signal, the master turns the output of this astable off when it is not giving a high itself, but this signal represents the ouput of the second astable if it had Pin 4 permanently connected high.

The output can provide a distinctive sound when passed through a buzzer, something like "dee-dee-dee-dee-dee-dee..." or a noticable flashing sequnece when passed through LEDs - ideal for a bike light or such. The number of short pulses per train and the length of the train is dependent on the values used for each stage.

Below is the circuit diagram for a standard astable bleeper with two stages and a master frequency of about 1Hz and a slave frequency of about 4Hz.

The entire circuit can be made using only one IC by using a 556 timer IC, which contains two identical 555 circuits. The circuit will be the same, but the pin numbers will be different.

A similar result can be obtained using the two astables and an AND transmission gate, but this requires more ICs and it also means that you may get out-of-sync signals. Imagine the second signal shifted slightly to the left in the above diagram - the output would have two short pulses at the start and end of the train, and two longer ones in the middle. This may not be disastrous, but if the shift "drifts" slowly over time it would not be ideal, as the number of pulses be cycle would vary. This shift could occur if the two astables were not prefectly in sync. The action of the reset pin "zeroes" the second astable each time the master goes high, preventing this creep happening.