555 monostable error.

Thread Starter

SeanV123

Joined Nov 12, 2020
68
Hello, I have a 555 timer set up in monostable operation a one it works perfectly. For a college assignment I need to show that when the trig pin is low the output pin switches on. Okay cool I'll use an oscilloscope. Here's my issue. I connect one scope probe to the output pin. But, when I connect the second scope to the trigger pin the output pin (connected to an LED) comes on. I think this is because when I connect the oscilloscope to the trig pin it allows the trig pin to be set to low hence setting the output pin high. Is there anyway I can connect the scope to the trig pin without setting the output pin high?
 

crutschow

Joined Mar 14, 2008
27,725
Yes, the probe is likely pulling the TRIG pin low, which triggers the 555.
But schematics are the language of electronics, and I can't give good advice without one of your circuit.
 

Thread Starter

SeanV123

Joined Nov 12, 2020
68
Yes, the probe is likely pulling the TRIG pin low, which triggers the 555.
But schematics are the language of electronics, and I can't give good advice without one of your circuit.
Yes, I should've provided some info. Apologies. If you could give any advice what would you say?
 

Thread Starter

SeanV123

Joined Nov 12, 2020
68
A scope probe shouldn't upset the circuit enough to trigger the timer. Show us a schematic first.
Okay I'll have to do that tomorrow, is there an app where I can create a schematic because I don't wanna draw one because I'm no artist
 

crutschow

Joined Mar 14, 2008
27,725
So how did you wire the circuit if you don't have a schematic?

Some use TinyCad, but I have not direct experience with it.
I use the free LTspice, which is also a SPICE circuit simulator.
 

dl324

Joined Mar 30, 2015
13,118
Okay I'll have to do that tomorrow, is there an app where I can create a schematic because I don't wanna draw one because I'm no artist
I have little artistic skill, but I can draw schematics on paper.

I use Eagle to draw schematics. The timer symbol they use is retarded, but you can create your own.

Here's your basic 555 one shot (with a modified timer symbol):
1614720363308.png
What we really need to know is how you have the trigger wired.
 

Thread Starter

SeanV123

Joined Nov 12, 2020
68
I have little artistic skill, but I can draw schematics on paper.

I use Eagle to draw schematics. The timer symbol they use is retarded, but you can create your own.

Here's your basic 555 one shot (with a modified timer symbol):
View attachment 231888
What we really need to know is how you have the trigger wired.
Yeah I will show you tomorrow sometime. It's basically a 10k ohm resistor going from the trig pin to a pushbutton switch and the other leg of the switch going to ground
 

dl324

Joined Mar 30, 2015
13,118
Yeah I will show you tomorrow sometime. It's basically a 10k ohm resistor going from the trig pin to a pushbutton switch and the other leg of the switch going to ground
What is the other end of the 10k resistor connected to?

This is Eagles idea of a 555 symbol:
1614721168150.png
They needed to consider that reset is usually tied to the supply. The symbol used by Nat Semi is better, but still not good.
 

Thread Starter

SeanV123

Joined Nov 12, 2020
68
What is the other end of the 10k resistor connected to?

This is Eagles idea of a 555 symbol:
View attachment 231889
They needed to consider that reset is usually tied to the supply. The symbol used by Nat Semi is better, but still not good.
10k resistor goes from the trig pin to one leg in the pushbutton and the other leg of the pushbutton goes go gnd
 
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