cuberoot

Hi!

I am using 3 monostable's to switch on an astable. Each monostable is individually triggered and switches the astable on for a different amount of time. When I connect each monostable seperately, the circuit works perfectly,but the moment I try to connect 2 or more monostables the astable starts to oscillate randomly when either monostable is triggered! What am I doing wrong??I am using the 555 timer for the monostable's and the astable!

Thanks!

Cuberoot

mechie

I wonder if you are just connecting the monostab.s outputs together - bad voodoo ops: . Try using a diode 'OR' gate to produce a coherent single "any one of three" output. How this works depends on what method of controlling the astab you have chosen.
IF a monostab goes logic 'HIGH' to start the astab then a diode will be anode-to-monostab, cathode-to-astab; reverse this if the opposite is true. Connect all the diodes together at the astab end (all leads WILL be either anodes or cathodes. You may need a pull-up or pull-down resistor at this point to one of the power rails (a few kOhms?).

Any help?

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cuberoot

Thank you!!! It worked!! You are a genius!!I dont know why I never thought of it myself!! Do you perhaps know the reason why you cant connect the outputs of the monostables together?? Im just curious!!

mechie

How to exlain without diagrams? lets try (sorry if it's pathetic as a result)...
If an IC output can be imagined for a moment as a switch; this switch forces the output to ground working against the output's load (a light bulb who's other terminal is connected to the positive supply) it's easy to see how the 'switch' controls the 'bulb'.
Add a second switch in parallel with the first and either will light the bulb, but both have to be 'off' to extinguish it: A simple 'OR gate' - no problem.
Some ICs can have outputs connected together like this (open collector circuits for example).

IF the switch isn't a simple one -- it now has a change-over action with one side to ground and one to the positive supply; the moving contact being the output (the same load as before)
A single switch like this works exactly the same - on-off -
A second switch with the 'outputs' connected together - switch either on and BOOM ops: expensive smoke!
A dead short across the power supply.
Lots of ICs have outputs like this - op-amps, logic ICs, blah blah.
(Luckily some have protection or limiting on their outputs - 555 timers and 741 op amps but it still isn't good and can be unpredictable).

The diode 'OR' gate I suggested prevents the outputs from 'seeing' each other - it effectively converts 'change-over' into simple switches :wink:

__________________
I need a memory upgrade ...
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