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Electronics => Beginners => Topic started by: Moriambar on August 27, 2018, 10:09:37 pm

Title: I need help with a "one hour timer" circuit
Post by: Moriambar on August 27, 2018, 10:09:37 pm

Hi there.

While reading the Art Of Electronics (3rd edition) I looked at the nice circuit at page 467, fig. 7.65 (https://imgur.com/gallery/EQxmrab) using an ltc6991 (http://www.analog.com/media/en/technical-documentation/data-sheets/6991fc.pdf) and some other components to create an "automatic shutdown" of the circuit after one hour from its power up.

I thought that was neat and tried doing my own version, only to make it go for half an hour. Only my circuit does not work at all. So, after three days of trial and failure I came here for support.

Here's what I have different:

• Instead of the LVC2G02 I have connected two NORs from a 74HC02N I had around
• Instead of the LVC1G04 I used one inverter from a 74HC04 I had around
• I used a 1M resistor as R5 (as suggested in the LTC datasheet) accurately measured among the ones I have
• I connected 880K of resistance, accurately measured (680k+ 2*100K series) as R4 – note that this is a measure I adopted in the later stages of the circuit, since nothing else worked – in order to be as much compliant to the datasheet as possible
• calculated and put in an 82K resistor as R3 to achieve my result
• used a different p-fet, an IRLIB9343 (http://www.irf.com/product-info/datasheets/data/irlib9343.pdf) which is the only p-fet I had around
• hooked up an LED to the fet drain to see when the cycle is through

Well after all of that the result is that well after 40~50 minutes, the circuit is still on and does not want to switch off!

Do you think I made any mistakes? If you think I should post images about what I did, I'll pretty 'em up and do it when I can (it's night here unfortunately).

Thanks to anyone even only for reading this

Title: Re: I need help with a "one hour timer" circuit
Post by: Signal32 on August 27, 2018, 11:19:07 pm

Hello,

Not sure exactly what is wrong but first step is to get the ltc6991 oscillating at let's say 1HZ frequency. Try to power a red led directly(through a resistor). Try to get that going and then you can continue with adding the MOSFTET and then the gates.

Signal32

Title: Re: I need help with a "one hour timer" circuit
Post by: Moriambar on August 28, 2018, 06:16:19 am

You're right... I should start simple: nice idea! I was so frustrated that I did not think about it.

EDIT: yes, starting it simple made me solve everything in around five minutes

Thank you!