NE555: circuit not working

ed varga

Part Number: NE555
Other Parts Discussed in Thread: CD4521B, CD4541B, CD4060B

we had an EE design a 450 to 500 sec. timer. We purchased the components from Digi-Key. Assembled one, does not work. EE wants more $, we lack trust. Is this a resource we can use to help us. We are ME's with some knowledge of electronics.

over 7 years ago

0 Ron Michallick over 7 years ago

TI__Guru**** 165326 points

Ed,

For such a long timing (one shot) period, consider all leakages as a limiting factor.

What values did you chose for your timing resistor and capacitor? Please share the digikey part number for the capacitor.

What is the failure mode? For example, "output never goes low (end of timing period)"

0 ed varga over 7 years ago

Prodigy 105 points

cap. is .47uf at 16 volts, digikey 445-8419-ND; resistor is 10k 5%, digikey CF14JT OCT-ND. failure mode, output never starts to time. As soon as power is applied Q2 turn on.

0 Ron Michallick over 7 years ago in reply to ed varga

TI__Guru**** 165326 points

Ed,

Time period is 1.1 * R * C = 1.1 * 10k * .47u = 5.17ms; this is 0.005 seconds
Is it true that you wanted 400 to 500 seconds (6 and 2/3 minutes to 8 and 1/3 minutes)?

Is your circuit closer to figure 9 or figure 12 in the NE555 data sheet?
Does Q2 on mean output (pin 3) is low or high?

0 ed varga over 7 years ago in reply to Ron Michallick

Prodigy 105 points

would like to send schematic, not sure how. Ed

0 Ron Michallick over 7 years ago in reply to ed varga

TI__Guru**** 165326 points

Ed,

Click this first.

Then this

0 ed varga over 7 years ago in reply to Ron Michallick

Prodigy 105 points

0 ed varga over 7 years ago in reply to ed varga

Prodigy 105 points

hello mr. michallick, the input power up to the R1 works. I have +10 volts DC. when connecting the reed relay to the 24 volt supply, that works.
what is not working is the timer. Q2 never turns off, as soon as power is supplied, Q2 is on. we have 10 of the NE555, and swap out two or three without any change. Do hope you can help We are a month behind starting production and still need to have PCBA made. Ed PS we are low cost tooling ( USA ) short run injection molders so if we can return the favor let us know.

0 Ron Michallick over 7 years ago in reply to ed varga

TI__Guru**** 165326 points

Ed,

Add a resistor (30k will do) from pin 2 to pin 8. The trigger input needs an external pull up resistor. There is no internal resistor on this pin.
Now timer won't stay output high. I believe timer won't start on power up, but if it does add resistor from pin 8 to 4 and add cap from pin 4 to GND. This will startup with RESET low then charge to RESET high.

0 Ron Michallick over 7 years ago in reply to Ron Michallick

TI__Guru**** 165326 points

If 10V is not low noise then a 10nF or 100nF cap from pin 5 to ground will help. This prevents VCC noise from causing shorter timing periods.
As drawn timing period is 1.1 * 10k *100uF = 1.1 seconds.

Do you want 7 minutes of on time (420 seconds)? If yes, I have some ideas.

0 ed varga over 7 years ago in reply to Ron Michallick

Prodigy 105 points

Hello Mr. Michallick, with your info, 1.1 seconds is a long way from 420 seconds. That said, our cap needs to be 38,000 uF. Our injection molds are done, so I do not see how we are going fit that in the unit. So please tell us there is a way out. The lamp needs 300 seconds to warm up and 120 seconds to kill the DNA. Ed

0 Ron Michallick over 7 years ago in reply to ed varga

TI__Guru**** 165326 points

Ed,

I found three logic oscillator with multi stage ripple counters. This will make long timing period possible with smaller capacitors

CD4521B

CD4541B

CD4060B

All these will work at 10V

0 Ron Michallick over 7 years ago in reply to Ron Michallick

TI__Guru**** 165326 points

Ed,

In case it is not obvious frequency of oscillator needs to be freq = 2^(n-1) / (7 * 60)

For example, if N = 14 then freq = 2^13 / 420 = 19.5 Hz for a 7 minute.

However, a latch is still needed to start and stop the CD4xxxB.

Here is an untested solution to use NE555 as the control latch and lets the CD4060B run for 7 minutes.

0 ed varga over 7 years ago in reply to Ron Michallick

Prodigy 105 points

thank you, ordering parts today, will confirm working or not, back to you in a few days. Ed

0 Ron Michallick over 7 years ago in reply to ed varga

TI__Guru**** 165326 points

Ed,

I used a breadboard to build circuit and it worked.
Cx = 0.47uF, Rx = 49.9k, Rs =226k for 19.5Hz
My cap measured 0.458uF, so it ran at 20.0Hz
On time was 6 minutes and 51 seconds.

0 ed varga over 7 years ago

Prodigy 105 points

Hello Mr. Michallick, in your Nov 09, 2017 reply you gave info on Cx and Rx can you send me the schematic . The Nov 02,2017 schematic you sent does not have those designations. Thank you, Ed PS we are still waiting for the CD4060B components from Digi-Key.

0 Ron Michallick over 7 years ago in reply to ed varga

TI__Guru**** 165326 points

Ed,

Cx, Rx, Rs are mentioned in bottom of page 3 of CD4060B data sheet. Nov 2nd schematic show only NE555 section of the solution. CD4060B section needs the three passives, VCC & GND connection, and a power supply bypass capacitor.

0 ed varga over 7 years ago in reply to Ron Michallick

Prodigy 105 points

thank you for all your help. if we can be of any help, please just ask. Ed

Clock & timing

Clock & timing forum

NE555: circuit not working