inverting delayed 555 output

Thread Starter

DonLduk

Joined Jun 12, 2017
16
Hello.
I want to invert a delayed 555 output pulse.

I'm triggering the 555 some 3 seconds after circuit power-on. Then I feed the signal to a NPN transistor (2N2222) to invert it. I need to have a ground pulse generated 3 seconds after circuit power-on.

However, with the following circuit (image attached) I'm getting a ground pulse from the transistor on circuit power-on, and then again after the 555 triggers 3 seconds later. So my load is triggered not from the delayed 555 but from the power-on ground pulse.

How do I prevent my transistor to send a ground pulse on power-on?

Screen Shot 2017-07-15 at 4.33.56 PM.png
thanks.
 

dl324

Joined Mar 30, 2015
11,258
what should it be?
i only need a 300ms pulse
R2 has nothing to do with pulse duration; it establishes base current, which we can't calculate because you didn't include supply voltage in your schematic. We also can't tell you what it should be because of the missing supply voltage.

Pulse duration is controlled by the timer; which we also can't calculate because your schematic isn't complete.
 

Thread Starter

DonLduk

Joined Jun 12, 2017
16
Hello, you are right, sorry for the incomplete information Find attached the complete diagram. Vcc is 5V.

As recommended by dl324 , I added a 33k and 100uF to pin4 on the right 555 to stabilize power. These values chosen so it is larger than Ra2 and C2 (longer than the pulse).

with this addition, sometimes trigger2 goes to ground after 3 seconds after power-on, but sometimes it triggers after immediate power on.

thank you for your support.

Screen Shot 2017-07-16 at 10.14.48 AM.png
 

crutschow

Joined Mar 14, 2008
25,262
what should it be?
i only need a 300ms pulse
The usual rule is to make that base current about 1/10th of the collector current for good saturation of the transistor as a switch.
You have that backwards with the base current about ten times the collector current.
Thus Rb1 needs to be no smaller than 4V/(0.5mA*10) = 80kΩ.
I added a 33k and 100uF to pin4 on the right 555 to stabilize power.
........with this addition, sometimes trigger2 goes to ground after 3 seconds after power-on, but sometimes it triggers after immediate power on.
Try connecting pin 4 of the left 555 to pin 4 of the right 555 (with the R and C delay).
 

dl324

Joined Mar 30, 2015
11,258
The time constant for the reset is too long and it's connected wrong. Put the resistor between VCC and Reset and the cap between Reset and GND. Connect the negative terminal of the cap to GND. 100k and 0.1uF would give you a reset of several 10's of mS which should be sufficient.

As shown, the reset capacitor polarity is reversed and polarized caps won't tolerate more than around half a volt of reverse bias.
 

crutschow

Joined Mar 14, 2008
25,262
........100k and 0.1uF would give you a reset of several 10's of mS which should be sufficient.
The 555 Reset pin has a low threshold voltage (basically one base-emitter drop above ground)
upload_2017-7-16_10-14-11.png
so you need a longer time-constant of a few hundred ms to get several tens of ms of Rset time.
 

EM Fields

Joined Jun 8, 2016
583
Hello, you are right, sorry for the incomplete information Find attached the complete diagram. Vcc is 5V.

As recommended by dl324 , I added a 33k and 100uF to pin4 on the right 555 to stabilize power. These values chosen so it is larger than Ra2 and C2 (longer than the pulse).

with this addition, sometimes trigger2 goes to ground after 3 seconds after power-on, but sometimes it triggers after immediate power on.

thank you for your support.

View attachment 131049
Try this:
Negative pulse after power-up delay.png
 
Last edited:

Thread Starter

DonLduk

Joined Jun 12, 2017
16
wow thank you very much for your responses.

I got it working with the transistors in saturation as recommended, and also the RC on Pin4 works flawless with 150k and 100uF .
 
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