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.
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.