Need help from an electronics engineer or technician with circuit problem.?

Question

I'm using a 555 timer set up as an astable multivibrator to drive a 74HC393 dual 4-bit counter. The outputs of the counter are tied together via 3 AND gates to create a hi output when the counter outputs are all high. The hi out of the and gates is in turn used to drive the clock of another 555 set up as a one-shot.

The output of the ANDs is also wired back to the CLR input of the counter via 4 inverters (possibly not necessary but I figured the propagation delay would ensure the pulse was long enough to clock the one-shot) so that the clock signal pulse into the clock of the 555 one-shot is of shorter duration than the designated T high set for the one-shot.

All of this works perfectly except that when power is first applied to the circuit the one-shot fires immediately and the counter initializes to a random number rather than starting at 0000 as expected. Even though the counter shows a random number when the circuit is turned on I can watch it count through a full cycle of 0-15 of 0000 - 1111 even though it isn't necessarily starting at 0000 (I don't really see the 1111 state since the pulse is too short due to the reset but it causes the 555 one-shot to fire so I know it is taking place).

Unfortunately I'm working with just a multimeter and LED indicators attached to the circuit for visual display of the outputs and don't have access to an oscilloscope.

I figure an experienced engineer might be able to give me an idea what might be going on and how to fix it.

I've got 16 years of experience as a technician but I've been out of the field for two years and my last two years of experience were as a test area supervisor without a lot of troubleshooting required so I may be a bit rusty. In any case I'm stumped for ideas at the moment.

Answer 1

I suspect your 555 one shot is firing at power up without getting full count pulse from the gates and also the counter is not getting reset at power up, but simply starting at a random count. If you want the counter to begin at a reset state, then either you take advantage of the false fire of the second 555 to reset the counter (instead of using the full count gate). Or build a separate power up reset generator that locks everything in a known state for some short period of time as supply voltage rises to a working level.



You really haven't defined what state you want the circuit to be in at power up.



By the way, Yahoo Groups has a very good place to delve into the details of such problems. It is Electronics_101. You are welcome to join.



http://groups.yahoo.com/group/Electronics_101/



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Regards,



John Popelish

Answer 2

Your ckt goes from 0 to 15, not to 16

1111 is 16!

It is a working method, but must be used with caution.

The 17th pulse 1111 is just a glitch; it never reaches full output because as it rises at some point it resets the circuit.

If that pulse was used somewhere else, other gates may not trigger



Hope this properly answers your question





Guru

Answer 3

I have a 2 year degree in EET. I have been in the field for over 20 years. I do work of a 4 year degree person without the pay plus maintenance. Less chance for advancement without the 4 year plus degree. I do enjoy electronics but if had to do it again I would stay away from manufacturing type jobs in the US. Outsourcing is causing them to diminish and keeping wages down. Go into something medical. There are 2 year programs in the medical field.