# will this timer/delay circuit works as intended with almost equal opamp inputs?

I just saw this video about a so called delay circuit using opamp in comparator configuration. The author mentions to use a pot as a voltage divider to feed the (+) input and the node between a series R and C to the (-) input. however the assembled circuit on the breadboard is missing the connection of the pot to ground (like the schematic I included bellow). so basically after some amount of time the comparator should see almost equal inputs thus the output is indeterminable. I hooked up the circuit using same values but a TL082 and I got white noise as expected, yet I can't understand does this circuit really outputs a steady low output? the SW1 is meant to restart the timing. does the pot and internal input resistance forms a divider? the author confirms that only two terminals of the pot are used in the comments. do i miss something else?

simulate this circuit – Schematic created using CircuitLab

the author confirms that only two terminals of the pot are used in the comments.

Then he's stupid.

This will not work as I see it working. You need to set a defined voltage at +Vin by using a potential divider and not rely on the giga ohm input impedance of the op-amp's input resistance as some kind of valid value.

Other problems: -

• The TL082 minimum recommended supply is 10 volts and not 9 volts.
• Input common mode range is guaranteed to be +/- 11 volts on a +/- 15 volt rail and if your supply rail is (say) +10 volts then the input range is from +4 volts to +6 volts.
• Output voltage range is guaranteed to be +/- 12 volts on a +/- 15 volt rail hence, on a + 10 volt supply, the output cannot be relied upon outside the range +3 volts to + 7 volts i.e. your LED will likely always be on. Typical limits might cause the LED to switch off but you shouldn't rely on typical limits because you'll get bitten.
• The author is probably relying on the input current. – Harry Svensson Apr 20 '18 at 9:41
• @HarrySvensson - it's a FET input device with worst case of 8 nA!! – Andy aka Apr 20 '18 at 9:47
• I was focusing on this part: "I hooked up the circuit using same values but a TL082". - I didn't bother looking up what the author actually used in the video. But apparently the author did not use the TL082. – Harry Svensson Apr 20 '18 at 9:50
• Maybe with a 800Gohm pot? – Dorian Apr 20 '18 at 9:51
• One way or another, some part of the spec will be contravened @H. – Andy aka Apr 20 '18 at 9:53