# Gating the output of a 4046 VCO?

Discussion in 'The Projects Forum' started by electronice123, Oct 10, 2008.

1. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
304
0
I am new to electronics and I need help gating the output of a 4046 VCO, the VCO is a 0Hz-10KhZ 50% duty cycle built from a 4046.

I need to be able to gate the output every 20 pulses or so. I would like to have a few gating ranges I could choose from like 5, 10, 20, 40.

I have been told this can be done with a 4040 counter, a few AND gates, and an inverter.

Can someone show me a schematic of how to do this? It would be very much appreciated!

Last edited: Jul 26, 2010
2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Yes you can do it!

Connect the output of the VCO to the clock input of the counter as to measure the output pulses. A 4024 counter is enough to count up to 40.

The output of the counter at

decimal binary

5 = 0000101
10 = 0001010
15 = 0001101
20 = 0010100
25 = 0011001

and so on

The easiest way to detect when the counter counts up to 5 for example is to put a two input AND gate on bit-1 (LSB) and on bit-2. Thus when the counter will count up to 5 the output of the AND gate will output a high logic and you can use it to drive what ever you want. Also, you will use this signal to reset the counter as to start counting from the beginning and count again up to 5. Note that the high logic on the output of the AND gate will go low when the counter will be reset, so you will need a RS flip flop to latch when the AND gate will output this brief pulse if you want your load to remain on. Then you can reset the RS flip flop anytime you want with the reset input and turn off your load.

This is the general idea, use AND gates for each combination of number of counts you like the load to be activated and then you can use switches on the output of the AND gates to choose which AND circuit (which number to count up to) will turn on your load.

3. ### IT_Guru Member

Nov 20, 2008
17
0
Any chance I could get a copy of the schematic you intend to use for this? I built one 20yr ago (1988) that is still going, uses only 5 external components but suffers from freq/temp drift. It swings 0 to 46Khz.

This URL is dead or I would not have bothered you: http://i51.photobucket.com/albums/f389/xbradkx/circuit123.jpg

Thanks!

Last edited: Jan 12, 2009