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Need a Timer Circuit to Drive Multiplexer 4051.Helpful Member! 

Ejaz (Electrical) (OP)
4 Dec 01 6:17
I need to build a Timer/Counter circuit to drive MUX 74hc4051. For the first 30 secs, the trigger inputs of MUX A,B & C are 'LOW (0)'. Then A goes 'HIGH (1)' for 30 secs and B & C remains '0'. After that A goes to '0', B goes to '1' and C remains '0'. Then A, B & C all become '0' for 90 secs. So, A & B both remain '1' for 30 secs and '0' for 150 secs. So, the total period for A, B & C is 180 secs.
C always remains '0'. During the first 90 secs, the INHIBIT of MUX remains '0'. After that, it goes to '1' and remains in that state for 10 mins. That means there will be no output from the MUX for 10mins. After that INHIBIT goes to '0' and the cycle starts again. Could anyone plz help me in designing the circuit! Thanks in advance.
melone (Electrical)
4 Dec 01 7:04
What other factors are involved, i.e. cost, space, temperature, etc.?  Otherwise, this circuit sounds VERY feasible.
nbucska (Electrical)
4 Dec 01 16:11
Give a timing diagr. I suggest ___===____===  format, etc.


Helpful Member!  MMAshwin (Computer)
4 Dec 01 18:59

Use a 555 chip as an astable with a period of 30 seconds.

Feed the output of that to clock a BCD out chip (i.e. as you clock it the ten ouputs 0-9 go high/low). An example of such a chip is the 4017.
See the following link for data sheet on a 4017

Use the 0 output as your first clock (A)
Use the 1 output as the second (B)
And use the Nth output to reset the counter so that on the next clock the 0 input goes high next and the cycle repeats.
Which output you use depends on how long afer the B output you want both to remain low

Your waveform would be
Clk __--__--__--__--__--__--__--__--__--__ 15 sec on/off
A   __----____________________----_________
B   _____----____________________----___
N   ____________________----_____________

As for the inhibit. Feed the same clock into a regular counter and AND the 4 & 8 output. When they are both 1 (i.e. 12) then 10 minutes has elapsed. Use this to enable/disable you mux.

Need any extra help, then email me with more details.

Any help ?, yes no let me know.


Ejaz (Electrical) (OP)
4 Dec 01 20:54
At present, I'm only concerned about cost and space. But it's not that important.
The Timinig Diagram is as follows:

INH_________========//=======_________  90s+10min+90s

  30s 30s      
A  ___===__________//______________===___  

B  _______===_______//_________________===

C  _________________//____________________
Ejaz (Electrical) (OP)
4 Dec 01 21:19
Hi Laffalot,

Thanks for the info. However, I have a question. Using a 555 to generate 30 sec pulses requires large caps and resistors. Is there any other way to do that? Thanks again!
MMAshwin (Computer)
5 Dec 01 0:23

If space is a limitation (big capacitors) then use a 4060 IC and a 32,768Hz (watch type) crystal.

The 4060 is a counter with a built in oscillator.
Check out the data sheet of it at:-

The final stage would output pulses every 1/2 second. You could feed this as a clock into a counter and again tap off at 30 sec intervals.

If the addition of that extra counter to get it to a 30 sec period is to much, then the 4060 built in oscillator can be run from an RC network. You could then choose an approprate RC combination so that the output of the 4060 changed every 15 secs.

Another method, if you program them, is use a PIC such as the 12C509. Its an 8 pin device with a built in oscillator. You can program it so that the outputs are A on 30 secs, A off, B on 30 secs, B off, inhibit on, wait, inhibit off  then repeat.

Find out about the 12C509 at :-

This would cut down on everything. Just program it and connect it to your multiplexer IC. Only one 8 pin IC !.

If neither of the above suit you. The you could still use a 555 as the frequency is dependent of the capacitance and resistance. There are some very small high value caps and also small (eg 1/8 watt) high value resistors.

Any help ?, yes no let me know.


Ejaz (Electrical) (OP)
6 Dec 01 22:47
Thanks a lot! I'll go for the 555 with 4017.
Ejaz (Electrical) (OP)
7 Dec 01 0:04
Well, I've got two more questions for Laffalot. You told me to AND output 4 and 8 of a Regular Counter to feed to the INHIBIT of the MUX. What kind of counter is this? And how to RESET it?
MMAshwin (Computer)
8 Dec 01 8:24

It could be any ripple counter which is clocked by a positve going wave form.Examples of which follow:-

You might want to try a 4060 14 stage ripple counter:-
(Mind you you'll only be using the first 4 bits)

or there is the 74LS90, 74LS92, 74LS93

Each has a reset line just check out there pinouts.

Any help ?, yes no let me know.


Ejaz (Electrical) (OP)
9 Dec 01 20:15
Thanks again!
Ejaz (Electrical) (OP)
14 Dec 01 4:03
Hi Laffalot,
I've bought the CD4060 counter but can't figure out how to make it LOW for the first 90 secs and HIGH for the 10 mins. If I use Pin 11 as the CLK input the circuit triggers on the negative edge of the pulse. However, if I use Pin 10 (OSC') and ground Pin 11, it triggers on the positive edge. The minimum no. of cycles that it counts is 8 (pin 7). The timing diagram is like this.

RST _=__________________________________________________

Pin7 (8 Cycles)

Pin5 (16 Cycles)

Pin4 (32 Cycles)

Pin6 (64 Cycles)

Pin14 (128 Cycles)

Pin 13 (256 Cycles), Pin 15 (512 Cycles) and so on.

I would appreciate it very much if you could tell me how to make it LOW for the first 90 secs after RESET and HIGH for 10 mins.
MMAshwin (Computer)
14 Dec 01 7:15

I'm going to do a schematic and get back to you. Please hold on.

Any help ?, yes no let me know.


Ejaz (Electrical) (OP)
16 Dec 01 23:16
MMAshwin (Computer)
19 Dec 01 5:57

Sorry this has taken so long but I've had a lot on.

The diagram below is the circuit

Where IC1 = 555, IC2 = 4060 & IC3 = 4017.

I've posted a completed solution at the following link, including timing waveforms and explainations. If I had the time I would have like to make it more complete but it should be enough to get you going.
Also forgive the website. It has had a total of 20 mins development time (created for this very purpose).

and follow the solutions link to timer.pdf

Any help ?, yes no let me know.


Ejaz (Electrical) (OP)
19 Dec 01 21:54
THANKS A L...O...T!!! I browsed through your homepage. Nice layout. But I couldn't get access to the details of the circuit saying I was forbidden to access. Keep up the good work!!!
MMAshwin (Computer)
20 Dec 01 8:12

Whats your email & I'll send it


Any help ?, yes no let me know.


jbartos (Electrical)
24 Dec 01 20:32
Suggestion: There appears to be some need for C for the MUX.
Ejaz (Electrical) (OP)
5 Jan 02 9:39
Hi laffalot! I've been on leave since Dec 20th and just returned. My e-mail is

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