How do I turn a VC- Switch into a VC+ Switch?

Thread Starter

sciengart

Joined Mar 25, 2020
36
The switch in this schematic connects negative supply to the trigger and activates the timing cycle. I want to know how, without changing the initial circuit, can I use a transistor to implement switch that triggers from positive supply instead.
Any help would be greatly appreciated as per usual!Resettable Monostable 555 - Works - AB.gif
 

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dl324

Joined Mar 30, 2015
10,536
Add a transistor wired as an inverter. Putting a HIGH on the base resistor will cause it to turn on and give a LOW on the collector. When the base resistor is at a LOW logic level, the collector will be at a HIGH logic level.

1585338256986.png
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
You mean like a not gate?
I tried this though and it didnt work before. Ill give it another shot, will be back with results.
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
The voltage at the node between r1 and the collector in the not gate, reads 6v upon button press, despite the led turning off. As a result the trigger does not go low:(
 

ebeowulf17

Joined Aug 12, 2014
3,274
The voltage at the node between r1 and the collector in the not gate, reads 6v upon button press, despite the led turning off. As a result the trigger does not go low:(
What are you using for the not gate? Did you use the simple 2N3904 circuit, or some inverter IC? How exactly did you wire it into your 555 schematic?

What LED are you referring to? There is none shown in your schematic.

It's hard to help troubleshoot a circuit without knowing what you've got.
 

dl324

Joined Mar 30, 2015
10,536
The voltage at the node between r1 and the collector in the not gate, reads 6v upon button press, despite the led turning off. As a result the trigger does not go low:(
Are you referring to the component designators on the schematic I posted? If so, that does nothing to the trigger input.
 
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ci139

Joined Jul 11, 2016
1,378
Add a transistor wired as an inverter. Putting a HIGH on the base resistor will cause it to turn on and give a LOW on the collector. When the base resistor is at a LOW logic level, the collector will be at a HIGH logic level.

View attachment 202598
yes , but perhaps to pin 2 (\(=\overline{SET}\)) while the pin 4 (\(=\overline{RESET}\))
 

Thread Starter

sciengart

Joined Mar 25, 2020
36
Add a transistor wired as an inverter. Putting a HIGH on the base resistor will cause it to turn on and give a LOW on the collector. When the base resistor is at a LOW logic level, the collector will be at a HIGH logic level.

View attachment 202598
SOLVED IT!!!!!
Nether of us seemed to notice that we were running r1 to 5v+ rather than to pin 4!!!
It works now!!!! New schematic to follow!
 
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