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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
By my math I should be able to just change the timing capacitor to 7uF with no other changes and correct for the mistake, and still be very close to my 4hr mark (within a few minutes iirc). I'll just need a good way of hitting 7uF, an uncommon value. I'm thinking either 2x 3.3uF + a 0.47uF all in parallel.
Way too hard. Just whack in whatever 6.8uF, or 10uF, or 4.7uF you have to hand, and then muck about with a resistor or two based on what time you actually get. This is on the basis that capacitors tend to be large, and uncommon in a junk box, whereas resistors are small cheap, good tolerance and more common.Check the timing on the oscillator output so you don't have to wait too long.
I finally built the circuit (on breadboard) and I used a 0.1uF Ct for a total count down time period of 3min 42sec. I tested it out with a stopwatch and it came to 3min 34sec, which allowing for the wide tolerances on the cap and timing resistors works out just fine. The binary multi-led display is pretty cool looking as well and works great. Thank you all for the massive help! I'll give it a try with the full value capacitors for the 4hr test and report back how it goes - I'm not expecting any issues as long as the oscillator will run well at the 0.57Hz mark.
UPDATE: I did the full term test and it ended one hour early. I checked the R and C values with my meter and they were all within spec. By using the formula for the internal oscillator and inputting the observed total counting period I was able to get the effective value of R out: a mere 75kOhms! Now, I measured the resistors to be 98kOhms but perhaps when combined with internal resistances in the IC some sort of R network forms lowering the effective resistance. I then recalculated how much capacitance would be needed to correct for the error and re-ran the test. Success! The timer counted down for 3hrs 58min. Now just to migrate from breadboard to PCB and build the unit.
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Simple RC Timer help
