# Burst of 40Khz

1. May 24, 2010

### cat1cat

hi. I have a project in hand. I have to make a 40 Khz continious wave in to a burst of signals.
for this i am using 2 555 timers . 1st astable circuit and the output goes to the triger of the monostable circuit and then the output of this circuit is ANDed with the 40Khz continious waves from other circuit. Will this whole set it give burst of signal? please help...

2. May 25, 2010

### vk6kro

No, that will not work.

The monostable will trigger again when it finishes the time-out because it is getting a continuous stream of pulses.

You could drive a counter with the 40 KHz pulses and then use the output of this to gate the 40 KHz pulses.

You can get crystals for 40 KHz and these would give much better stability than an astable.

If you wanted to consider the counter idea, have a look at the data sheets for the CD4020 CD4040 and CD4060 or the 74HC4020 etc.
You can string these together to get division by powers of 2.
Divisions like 8192, 16384 are available so you could get square wave outputs of 4.88 Hz and 2.44 Hz, so you could get bursts of 40 KHz for 0.1024 seconds or 0.2048 seconds

Another chip worth knowing about is the HEF4521 which will divide by up to 16777216 which would give a period of 419 seconds from your oscillator but you can get shorter times than this. Outputs of 2^18 to 2^24 are available. 40000 divided by 2^18 is 6.55 seconds, so you could get bursts of 3.28 seconds.