Trigger 555 with positive pulse

[ghall426]

I need to trigger a 555 timer with a positive pulse instead of the negative pulse required. Using a 2N2222 transistor, how can I calculate the base resistor and collector resistor needed? I will have the emitter tied to ground.

Thanks.

Greg

 

[ghall426]

One more thing

By the way, I tried using the circuit negative triggering as the 555 is designed but the trigger was so sensitive it would trigger by the touch of my finger. I tried a 200 ohm pullup resistor, but my finger could still trigger it. So I am assuming the transistor idea is the way to go and change my trigger mechanism to a positive pulse.

Greg

 

[audioguru]

With a 200 ohm pullup resistor from pin 2 to the positive supply your finger must be 100 ohms to ground (you are standing in a bucket of salt water?) to trigger a 555, or your finger picked up a strong radio station nearby.

Have you ever seen the shielded cable used for microphones and stereo components? It stops pickup from radio stations and mains hum.

 

[kchriste]

how can I calculate the base resistor and collector resistor needed?

A rule of thumb for making a saturated transistor switch is that the base current should be 1/10 of the required collector current. So,.... If your input voltage is the same as your output, the base resistor is 10 times the resistance of the collector resistor.
For your 555 circuit, try a 10K on the base and a 1K on the collector. A 2n3904 or 2n2222 should work fine as will almost any small NPN transistor you might have around.

I tried a 200 ohm pullup resistor, but my finger could still trigger it.

The 555 doesn't need much "time" to be triggered so the large pulse of current accross the 200hm: most likely came from your body capacitance and static charge. To guard against these types of triggers you can shield it, like Audioguru said, or use a lowpass RC network on the input to the 555 to filter out the highspeed pulses.

 

[ljcox]

Why don't you use a Monostable instead of the 555? Then you only need a capacitor and a resistor since Monostables are edge sensitive.

 

[Hero999]

Don't bother doing any calculations, just use the values in the diagram below.

 

[simontay1984]

Don't bother doing any calculations, just use the values in the diagram below.

Would that circuit be suitable for triggering a 555 using the output from an amplifer.
I am trying to build a sound to light circuit for a strobe light. Here is my original post: https://www.electro-tech-online.com/threads/strobe-sound-to-light-pulse-trigger-circuit.39896/