Mono + Astable 7555 Timer Output Troubleshooting

Thread Starter

Will_f1

Joined Aug 16, 2021
5
My application is to build a reusable Fuel Injector cleaner / tester, as described here http://hackaweek.com/hacks/building-an-electronic-fuel-injector-tester/

What I’m doing seems like a reasonably well discussed circuit, however this issue has me stumped.

I’m trying to trigger an astable circuit from a one-shot monostable circuit, holding a pulse for a period of time (adjustable). If I isolate each timer, they work as expected. When I trigger the astable circuit from the monostable circuit, I get one pulse cycle and no further.
Unfortunately I don’t have an oscilloscope (not sure I need one), measuring the voltage at the output of U1, starts at 11.2v until 1 cycle is complete, then drops to ~2.4v (Astable output pin hits 0v) until the monostable timer completes.

I’ve chosen the timer values to try best represent fuel injector timings:
  • Mono 1s & 5s
  • Astable 1s/1s & 1mS/1mS

I’m using the 7555, given it’s a more modern implementation and seems more “robust”.

There could be a better way of doing this, electronics isn’t my strong suit and I’m open to recommendations. One solution I pondered was to add a transistor between the two circuits.

Apologies, the schematic isn’t very well presented, I used the best tool I had at my disposal.

Mod: lightened your image.E


5921047B-9048-4018-A9FF-FB4D365F4A32.jpeg
 

LesJones

Joined Jan 8, 2017
3,523
U2 is not connected as an astable. The triggert pin should not be connected to ground. It should be connected to the top end of the timing capacitor. I am guessing that the pin you have labeled CTL is the control voltage pin. That should not be connected to ground. It should be left floating or connected to ground VIA a capacitor. (10 nF for example.)
Your schematic drawing is fine. (I normally just draw them on paper and scan it.)

Les.
 

Thread Starter

Will_f1

Joined Aug 16, 2021
5
Thanks for the response,
I drew reset and control in to try make it complete, practically I’ve got them floating given I’m using the 7555.
That trigger on U2 was a typo sorry, its connected to top of timing capacitor.

Photo of breadboard below …B7E1827A-D36F-4C6B-AD8A-AB24A9711B78.jpeg
 

LesJones

Joined Jan 8, 2017
3,523
Reset is normally connected to the positive supply as shown in your schematic. This is probably more important with the CMOS version of the 555

Les.
 

AnalogKid

Joined Aug 1, 2013
9,408
R5 and R6 are *way* too small in value. See the 8555 datasheet for recommended value ranges. Also, with a 1000:1 ratio between R5 and R7, the voltage at the Threshold pin never will go low enough to affect that input.

You can delete R8.

Powering one 555 from the output of another 555 is not a good idea. Better to continuously power both devices, and use the Reset input to control the second one.

In the schematic you copied (in your link), pin 4 is floating. This is a serious error. Leaving pin 5 floating should not affect circuit operation, although the datasheet recommends a noise filter capacitor to GND.

You show the U2 Trigger input connected to GND. This is an error.

ak
 
Last edited:

eetech00

Joined Jun 8, 2013
2,611
Here's a circuit that will do what you want.
I don't know what you intend to drive with the output but I've designated it "Flash".
I've shown NE555 but you can use 7555 as well. See below.
I would also add a 15-18v zener diode across C9.

1629299972594.png
 
Two comments:

1) the 7555 is not the same as the 555. Close, but not the same.
2) The horizontal power rails have breaks in them. The blue and red lines.

Not sure if either issue is a problem.
 

Thread Starter

Will_f1

Joined Aug 16, 2021
5
Simply moving the output from VDD to Reset (and connecting VDD to power) worked a treat.
Having said that, understand the circuit isn’t ideal. I’ll give eetech circuit a try on the weekend. Really appreciate the efforts.
 

Thread Starter

Will_f1

Joined Aug 16, 2021
5
Put together eetech00’s design today. I had some strange timings with a 10u cap, so changed to a 22u and worked a treat. Will go with that design.
Thanks again.
 
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