1
\$\begingroup\$

I have a PCB for a 555 monostable timer circuit as shown below:

enter image description here

For the most part, the components are clearly labeled (I think the round circle with a + inside denotes a capacitor, but please correct me if I'm wrong). The only component I can't quite identify is the one called "Preset"...

It has 3 pins, one on top and two on the bottom. The top pin is connected to R1 and eventually connects to the VCC and Reset pins on the 555 timer as well as the positive voltage source. The bottom left pin of the so called "Preset" component connects to the Discharge pin of the 555 timer. The bottom right pin of the "preset" component does not seem to connect to anything at all, so it surprises me that it's even there to begin with....

What should this "Preset" component be and what is it's role in the circuit? How does its value affect the behavior of the timer circuit?

UPDATE

Here is a look at the backside of the PCB. I flipped the image horizontally so that the left side of the image of the back PCB aligns with the left side of the image above. Hopefully, this will help to identify how the circuit is layed out:

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Can you show the cooper side of the PCB ? \$\endgroup\$ – MAC Feb 15 '15 at 17:34
  • \$\begingroup\$ @MAC: Good idea! I'm uploading the backside of the PCB as we speak. \$\endgroup\$ – Paul Feb 15 '15 at 21:58
3
\$\begingroup\$

Here is the most probable schematics:

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
  • \$\begingroup\$ @Paul, your PCB backside fits the schematics as far as I can tell. The Preset resistor (Rtime on my schematics) sets the duration of the pulse together with C1. \$\endgroup\$ – MAC Feb 15 '15 at 22:05
0
\$\begingroup\$

I expect that the "preset" component is a potentiometer. The terminal on the rounded end will be the wiper (moving contact), while the other two terminals are the ends of the resistance element.

Do you have a schematic diagram and/or parts list for this? That should clearly show what this component is.

Varying the pot will likely vary the time of the monostable.

\$\endgroup\$
  • \$\begingroup\$ Unfortunately, no schematic or parts list is available for this PCB. I've scoured google for it to no avail... \$\endgroup\$ – Paul Feb 15 '15 at 17:15
  • \$\begingroup\$ I posted an image the hackside of the pcb. Does this clarify things? \$\endgroup\$ – Paul Feb 15 '15 at 22:13
0
\$\begingroup\$

The resistive connection between the Preset (7th terminal) and the Vdd (or capacitor and ground) , would be defining the time intervals of the vibrator. Therefore, I also think that it is a variable resistance.

What I also suspect is that R1 and this pot are in parallel, so if you want to have a fixed time interval, you can choose a resistance, but if you want to modify it; you can stick to a variable pot. However, you should provide a complete schematic to be sure. For example, where is the other pin of the R1 is connected?

\$\endgroup\$
  • \$\begingroup\$ I have no schematic, but i posted an image of the backside of the pcb. Hopefully this will help. \$\endgroup\$ – Paul Feb 15 '15 at 22:11
  • \$\begingroup\$ As far as I can see, if you have a potentiometer there (as suggested) that defines the time constant together with the capacitor, which is connected to the pot together with the 7th and 6th terminal of 555 timer. It is similar to the schematic here electronics-tutorials.ws/waveforms/555_timer.html (please see monostable 555 timer) \$\endgroup\$ – user66802 Feb 15 '15 at 22:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.