Hello. I'm trying to design a 555 timer with duty cycle than 50% and, for some reason, it flashes once and then shines dimly continuously. The circuit is included. I'm aiming for a .2 sec pulse and 1.4 sec off. I've designed this in TINA, but the circuit doesn't want to cooperate. Only breadboarding it revealed the situation previous. I've read and studied what happens in astable timers, but nothing has helped. Can anyone solve this? Thank you.

 

I suspect the 3 Meg resistance is too high.

Bob

 

Offhand I see no other problem with the schematic except the Reset line is not connected (no dot).
Double-check all the connections on the board.

 

To start, shift some of the impedances. The reverse leakage current through the 1N4001 can be greater than the design current through the 3 M resistor. Try this:

R1 = 26K
R2 - 300K
C1 = 10 uF
C2 = 10 nF

Is the CA555 a bipolar of CMOS part?

Does the 555 drive the LED directly, or though a driver device such as a transistor? If the former, you can arrange things to eliminate D1.

ak

 

To start, shift some of the impedances. The reverse leakage current through the 1N4001 can be greater than the design current through the 3 M resistor. Try this:

R1 = 26K
R2 - 300K
C1 = 10 uF
C2 = 10 nF

Is the CA555 a bipolar of CMOS part?

Does the 555 drive the LED directly, or though a driver device such as a transistor? If the former, you can arrange things to eliminate D1.

ak

The 555 drives the led directly.

 

To start, shift some of the impedances. The reverse leakage current through the 1N4001 can be greater than the design current through the 3 M resistor. Try this:

R1 = 26K
R2 - 300K
C1 = 10 uF
C2 = 10 nF

Is the CA555 a bipolar of CMOS part?

Does the 555 drive the LED directly, or though a driver device such as a transistor? If the former, you can arrange things to eliminate D1.

ak

The 555 is bipolar. So, you're saying the leakage current diode shunts the 3M resistor so it doesn't discharge through pin 7?

 

The 555 drives the led directly.

The standard 555 astable circuit has a minimum duty cycle of 50%. That is, the output always is high longer than it is low. If you are driving an LED by pulling it's anode end up to (approx.) Vcc, you cannot get a short blink followed by a longer off time without a trick.

The trick is the diode. It effectively "shorts out" timing resistor "Rb" so the discharge time now can be shorter than the charge time. With this trick, you can get duty cycles less than 50%. However ...

If you change the LED connection such that the 555 is pulling the cathode end down to (approx.) GND, then the long part of the output duty cycle when the output is high is the off time. Now you can delete the diode and use the standard equations to get a 12.5% on (low) time.

ak

 

The standard 555 astable circuit has a minimum duty cycle of 50%. That is, the output always is high longer than it is low. If you are driving an LED by pulling it's anode end up to (approx.) Vcc, you cannot get a short blink followed by a longer off time without a trick.

The trick is the diode. It effectively "shorts out" timing resistor "Rb" so the discharge time now can be shorter than the charge time. With this trick, you can get duty cycles less than 50%. However ...

If you change the LED connection such that the 555 is pulling the cathode end down to (approx.) GND, then the long part of the output duty cycle when the output is high is the off time. Now you can delete the diode and use the standard equations to get a 12.5% on (low) time.

ak

I see, but is what I mentioned about the leakage current correct?

 

I see, but is what I mentioned about the leakage current correct?

Shifting the capacitors worked. Current leakage had to be the explanation of why the original circuit didn't work.
You know your stuff. Thank you!

 

Shifting the capacitors worked.

Do you mean replacing the capacitor?

 

Do you mean replacing the capacitor?

Changing the capacitance and using the recommended resistors. Excellent!

 

I breadboarded your original design and it worked fine. The component values were not out of range but it does depend on the quality of the components, mainly the capacitor.

 

I breadboarded your original design and it worked fine. The component values were not out of range but it does depend on the quality of the components, mainly the capacitor.

They're quality polarized caps.

 

The 555 input currents, and the diode and capacitor leakage currents, are in the datasheets. With this information, plus component value range information in the 555 datasheet, you can calculate the worst case circuit conditions.

ak