# 555 Timer output problem

Discussion in 'The Projects Forum' started by Jack Kotze, Mar 25, 2009.

1. ### Jack Kotze Thread Starter New Member

Mar 3, 2009
7
0
I need help with the output of a standard astable 555 timer circuit.

Led2 constantly glows dimly when the output is low but does brighten when the output goes high. Led1 for some reason always stays on, even when the output from the 555 is high. Led2 works correctly if I disconnect Led1.

I thought that my resisitor values may be to low? I calculated the resistor values to give me 20mA through Led1 at 2V, 15mA through Led2 at 1.9V and 10mA through the optocopler at 1.18V. Are my calculations correct? Have a placed the optocoupler protection diode (D) correctly?

Any help will be appreciated.

File size:
2 KB
Views:
38
2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
You need to move R2 so that it is in series with LED2, and drive the node between LED1 and LED2 with the output of the 555. Recalculate the resistor values to get the correct currents. Keep in mind that a bipolar 555 will only swing to within about 1.4V to 1.7V of the positive rail.
You don't need a protection diode for the opto device unless your input is cap-coupled (capacitor between 555 output and LEDs).
An optocoupler protection diode, when needed, should be anti-parallel with the opto LED.

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Think in terms of when the output of the 555 is low. You have two LEDs in series with R2 parallel to LED2 and the diode (which is not needed). Basically what Ron said.

File size:
2.1 KB
Views:
80
4. ### Jack Kotze Thread Starter New Member

Mar 3, 2009
7
0
Thanks Ron

I'll remove the diode, move the resistor and try again.

Just to be sure that I'm understanding the many 555 tutorials out there, when the output is low the voltage at pin 3 is 0V and about 1.7V less than the supply when the output is high?

5. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Take a look at the drawing I provided, it is a darlington pair output, usually it is around 1.4, depending on current it runs between 1.2 to 1.7V (with a heavy current draw, not the usual state).

File size:
10.6 KB
Views:
81
6. ### Jack Kotze Thread Starter New Member

Mar 3, 2009
7
0
So then it's better to calculate my resistor values based on Vcc minus 1.4V?

7. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I'd say so, unless you are going to pull 200ma, the max this chippie will deliver. Generally it isn't an issue for LEDs, since most of them drop around 2.0V or more. Older LEDs, stuff made in the 80's, can drop as low as 1.5 though.

8. ### Jack Kotze Thread Starter New Member

Mar 3, 2009
7
0
Thanks, Bill. I know what to do now.