Electronics > Projects, Designs, and Technical Stuff
Need some help with a 555 timer circuit
Pizzashape23:
--- Quote from: ozcar on August 11, 2020, 07:04:17 am ---
--- Quote from: Pizzashape23 on August 11, 2020, 05:46:47 am ---Would the values of the resistors in the edge triggered part of the 555 circuit have a big impact on the end result?
--- End quote ---
If you make R2 too low (relative to R3), then the trigger input would be low all the time the button is pressed. That wouldn't work for what you want.
--- End quote ---
That makes sense for that configuration, but what if i was to put R2 up to +12v (instead of connected to R3) and make both 10K, would this be a better configuration? I have seen both online but was curious if one was better than the other, for me anyway it's two less items on the BOM (I am already having to order some 10k resistors)
Circlotron:
--- Quote from: Pizzashape23 on August 11, 2020, 06:34:44 am ---That won't work, As stated in my question the output pulse width needs to be constant even when the input pulse is longer than the desired output. The input is a user activated button so in making the circuit edge-triggered i was hoping to eliminate that source of error.
--- End quote ---
When I said the trigger input I meant the actual trigger input pin on the 555, not where the pushbutton connects to the cct. As long as the CR network on the input makes a shorter pulse than the 555 output pulse.
Pizzashape23:
Sorry, your answer makes much more sense now, thanks
Ian.M:
One thing you haven't considered is switch wetting current. Unless otherwise specified, for reliability its good practice to have a minimum of 10V across a switch (or other contacts) when open and minimum 10mA through them at the moment they close to provide enough energy to break through any surface contamination that has built up. Of course this isn't always possible e.g. 5V logic circuits, so one does the best one can.
Pizzashape23:
The entire circuit runs on 12v, if that is what you meant, but I'm not sure about the current through the input button. The relays will be at the same 12v and one should be switching around 200ma minimum while the other up to 40A. The circuit runs off a 24v to 12v converter module capable of delivering 2A, so current shouldn't be an issue. It is quite possible I'm wrong, this is the first time i'm hearing about wetting current, so please let me know.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version