555 Outputs Conflicting (I think) - hellllp

[Raven Luni]

Greetings,

I've built most of the circuit for my M.U.N.T.E.R. project (diagram here: http://www.stonewolfsoft.co.uk/temp/munter01.gif )

I've ran into a bit of a problem though. The presence of the 2nd 555 for resetting the counter when it reaches 8 seems to completely bugger the master reset circuit.

First of all, there are a couple of minor differences in the way I've built it than in the diagram. Everything up to the 1st half of the programmer circuit has been built (the 4 ICs on the right havent been added yet). The 193 on the left is actually a 74HCT193 (all I could get) and instead of the Q8 pin going back to the reset, the output from the 555 is connected there instead (and neither of the 100nF capacitors to ground are there either).

What happens is that the master reset wont even trigger (and neither does the counter reset for that matter). If I take the second 555 out of the socket, the master reset works fine. The only other thing of note is that the routput to the reset pin is also connected to an LED via a 330 resistor.

All ICs except for the master reset 555 have 100nF bypass capacitors.

Almost forgot, the timing RC values for the 555 and 556 ICs in the programmer and audio interface are all 10K and 100nF and not the ones in the diagram.

 

[Raven Luni]

Here is the updated schematic with all the correct values and connections:

 

[Raven Luni]

Hmm - its almost as if the high state from the master reset is being pulled low through the diode. That cant be right can it? Can an output sink another one like that? Maybe if I add another diode then we'll find out........

 

[(*steve*)]

Hahaha, yeah. The 555s have their outputs connected together via a diode. If the outputs are set the right (wrong) way, current will flow between the supply rails via the two 555's outputs and the diode.

You need to contemplate the possible states and what is supposed to be happening in all of them.

If some states are illegal you need to determine that if you get into them that something will get you out and that nothing bad will happen.

 

[Raven Luni]

Much better! A diode from the 555 output to the reset solves the problem (at least it did several hours later after I found the shorted 1K resistor on the input side and the fried transistor). Now just the crosstalk on the inputs to fix - was it a stupid idea to put the ground capacitors on the op amp outputs? (it worked before but probably because I had done something else stupid at the time...)

Remind me why this is such a fun hobby again?

 

[BobK]

Remind me why this is such a fun hobby again?

Because it feels so good when you stop (i.e. solve the problem?)

Bob

 

[Raven Luni]

nope (I was talking about electronics in general) - nothing fun about debugging especially when youre out of your element (its enough of a pain doing it in software)