Simple 5Khz timer/tickle circuit - Page 1
Forum section: electronics / beginners

author: skymoco
author_rank: Contributor
timestamp: June 26, 2014, 11:44:16 pm
content: I'm trying to build a simple circuit that outputs a 5Khz signal with a 1% duty cycle. Attached is a simple circuit from a website (I'm pretty much just following along what he did). It seems incomplete, or maybe just not complete for a novice. There is no indication of the power in, nor is it clear to me what the bottom line is about. There is a dot on one end and a line that goes to nowhere on the other.I just need a simple version that I can breadboard. Following this diagram is becoming an exercise in frustration. Part of this is to learn, so I don't want to just go buy something. But I would be happy to provide enough via paypal to give someone a six pack of Toohey’s for a little help.Here is the link to the entire page.http://www.cooperman.talktalk.net/files/28.htm

author: tautech
author_rank: Super Contributor
timestamp: June 26, 2014, 11:53:55 pm
content: Download a 74HC14 data sheet and all will become clear.The key is it is a Schmitt trigger, anyone will do.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: June 27, 2014, 03:15:59 am
content: The bottom "terminal" should go to ground. That's a real poorly drawn schematic you found!On the HC14, pin 14 must be connected to the positive terminal on your supply (2v to 6v for the HC part) and pin 7 to ground. You can see that on the datasheet. EDIT: quick survey suggests the Fairchild datasheet is pretty good.More "Gotchas!":A small capacitor called a "decoupling capacitor" or "bypass capacitor" should be connected between pin 14 and 7, and placed physically close to the HC14. This powers the chip while it's switching. It takes some time for the power to get from your power supply to the chip due to the inductance of the wires. This is typically 0.1uF.Unused inputs must be tied high or low or bad mysterious stuff may happen even though you're not using those gates. So pins 9, 11 and 13 (assuming your using the pin 1, 3 and 5 inverters) should be attached to ground or +V. You can do other interesting things with them if you want like paralleling them with your final inverter to boost current. But for now just ground or vcc (v+, power +, whatever you want to call it).

author: skymoco
author_rank: Contributor
timestamp: June 27, 2014, 03:43:46 am
content: Ah, maybe that's why my simple connection didn't work the way I expected, I didn't ground the unused inputs. I have a scope that I use to tune servo motors and do basic stuff with.Anyway, will fix up my circuit and report back.

author: tautech
author_rank: Super Contributor
timestamp: June 27, 2014, 03:57:39 am
content: Quote from: skymoco on June 27, 2014, 03:43:46 amNormally one is advised in a good datasheet just what to do with unused inputs.Youmusttie them H or L unless datasheet says otherwise.EditTipDon't be temped to download the concise(small) versions of datasheets, much valuable info is missing from them.

author: skymoco
author_rank: Contributor
timestamp: June 27, 2014, 04:09:06 am
content: Upon further examination I have made more errors that I thought. I purchased 74LS14's and not 74HC14's like I thought. I assumed they are interchangable, but I understand from my reading they are not.Since I have the 74LS14's in hand, are there circuits that will do the same thing?

author: tautech
author_rank: Super Contributor
timestamp: June 27, 2014, 04:16:15 am
content: Quote from: skymoco on June 27, 2014, 04:09:06 amAs I understand it the HC is high speed Cmos and the LS is low power Schottky.Check datasheets for Vmax etc., they should do the same thing

author: TerminalJack505
author_rank: Super Contributor
timestamp: June 27, 2014, 04:45:17 am
content: LOL. I think I just realized why you refer to this as a "tickle circuit." I've never heard of such a circuit. The first half of the circuit, however, is what's known as a multivibratorso I'm guessing there was something lost in translation at some point.(The second half (from the 56pF cap onward) is for adjusting the pulse width of the output.)This circuit should work with 74LS14s--just barely. The LS series outputs can only go as high as 3.4V and source only 400uA so it will be a stretch, though.