Relaxation Oscillator

I have a MM74HCT14 (inverting Schmitt Trigger) being used as a Relaxation Oscillator feeding a 74LVC08A (AND-Gate).
I'm trying to get about 0.5Hz by using a 82k ohm resistor and a 33uF
capacitor, however, it doesn't appear to be working.
I'm wondering if the Schmitt Trigger can't handle the values I used. If this is the case, why?
Any assistance will be greatly appreciated.
Thanks in advance.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
BostonPeter wrote:

What kind of cap is it? What is its voltage rating?
If the cap's leakage is high enough, it may never charge far enough through the 82k resistor to reach the gate's input threshhold.
--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
------------------------------------------------------------------
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In article

I do not work with this often enough so that the part numbers read a bell. The time constant is 2.7 seconds.
Depending upon the circuit, the actual period can be greader or smaller than half the time constant by a factor of up to about five or so. It should work anyway if not at your desired frequency. What happens when you change the resistor and/or the capacitor?
If you have a scope, you should be able to track it down. Even a D'Arsonval voltmeter should be fast enough if you do not have a scope.
--

Sam

Conservatives are against Darwinism but for natural selection.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I don't have a scope handy but the voltmeter shows approximately 1.6 volts without oscillating.
If I short the 33uf (i.e. ground the input side of the invertor), it appears the AND gate that folllows is enough to turn on the output full and light the LED (AND gate goes through a 300 ohm resistor going to an LED).
Thanks

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Is the cap polarized? Perhaps try a smaller non-polarized cap.
George H.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I believe the problem is solved but I don't fully understand why.
My first step was to change the resistor/capacitor combination. Initially I thought the low value resistor (82k) was too low so I increased it to 1M and changed the capacitor to 2.2uF.
Unfortunately this still didn't work but I noticed the LED would light when probing around the inverter. This indicated it could be a bad connection so I reflowed the solder.
Afterwards I noticed the LED would light at times if the inverter pin was gently touched with a piece of wire. This led me to believe there was too much something on the etch; possibly too much inductance.
So I removed the capacitor and soldered it directly to the inverter pins with short bus wire. Moving it from the original 1.5" (using 10mil etching) away to within a 1/10th of an inch seemed to solve it.
Can someone explain why this occured? Was the long (1.5") etch causing this issue? If so, how can I prove this mathematically?
Thanks to all who contributed to this problem and will help explain my question.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Do you have any test gear? DMM and/or 'scope? By 'etch' do you mean a PCB trace? Are you making your own boards?
There are lots of potential errors. With a DMM/ Ohmmeter you can see if all the traces are correct... resistance is near zero where it should be and over 10 Meg ohm between traces.
What kind if flux did you use? did you clean it off? I've seen resistances between traces of ~ several meg ohm due to non-rosin flux.
George H.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.