Trying to understand PI metal detector circuit - Page 1
Forum section: electronics / beginners

author: macross
author_rank: Contributor
timestamp: April 05, 2015, 09:42:15 am
content: So i'm trying to analyse how this pulse induction metal detector circuit is working. What i don't understand is why 555 timer is not driving IRF740 directly, but instead it's driving BC807 first. Also i have i question about those two op amps. I think that the first op amp is configured as differentiator or am i wrong? As for the other i have no idea maybe someone could explain it to me?

author: Moseven
author_rank: Newbie
timestamp: April 05, 2015, 12:06:18 pm
content: It might not be the best circuit.Even for other languages, a capacitor should be written as 10µF instead of 10mkf. Resistor R11 should be 1M instead of 1m.The IRF740 has a large input capacitance. Because of that, for higher frequencies, the driver circuit for the gate should be low impedance. Perhaps that is what the BC807, the 33 ohm and the 150 ohm resistors are doing. But perhaps it is a fault, because there used to be a transistor there.The first opamp has a capacitor at the negative input, and a voltage divider at the positive input. That means the negative input is tuned towards the positive input. With the 1M, the gain is very high. It amplifies the input signal to full clipping digital output.The third seems to be the detection level or gain. I'm not sure. The capacitor C5 reduces the input signal, but OP2 does not have a feedback resistor, and pushes positive pulses into the transistor.Perhaps the transistor T3 is like a diode in a AM receiver.Here is another version :http://oi49.tinypic.com/b71iet.jpgThe low resistors are for enough current for the coil. Perhaps it was replaced with a mostfet without thinking about the currents.Here is a movie with that circuit :And here a project with it :http://radioskot.ru/publ/metallopoisk/metalloiskatel_pirat/16-1-0-480

author: macross
author_rank: Contributor
timestamp: April 05, 2015, 12:55:12 pm
content: Quote from: Moseven on April 05, 2015, 12:06:18 pmThanks allot for information!

author: wraper
author_rank: Unknown
timestamp: April 05, 2015, 01:01:18 pm
content: Quote from: Moseven on April 05, 2015, 12:06:18 pmSeem to be some Soviet/Russian circuit with part names redrawn to the western substitutes. mkf (in Cyrillic) was indeed the usual way how to write uF, the same was written on the actual capacitors.

author: Andy Watson
author_rank: Super Contributor
timestamp: April 05, 2015, 01:08:12 pm
content: It is slightly more difficult to make the 555 produce pulses that have less than 50% duty cycle. I assume that this application requires a low duty cycle pulse - the extra transistor is simply inverting the pulse to obtain low "on-time".

author: Zero999
author_rank: Super Contributor
timestamp: April 05, 2015, 06:58:40 pm
content: Why use the TL074 when only two op-amps are needed? Surely thr TL072 is cheaper, more compact and uses less power.If you must stick with the TL074, I'd be tempted to replace the 555 timer oscillator with a Schmitt trigger oscillator made from one of the spare op-amps in the TL074.

author: stitch
author_rank: Regular Contributor
timestamp: May 19, 2015, 04:54:40 pm
content: BC807 and IRF740 bear a resemblance to a ramp generator.

author: stitch
author_rank: Regular Contributor
timestamp: May 23, 2015, 04:20:24 pm
content: Yes, it's a ramp generator. BC807 sources a constant current to linearly charge the gate of IRF740. If it were not for the constant current, the gate would charge exponentially like a capacitor in an RC circuit. The question now is why do you want a ramp wave output from IRF740.http://www.csun.edu/~ih20409/SCHOOL1/RAMP%20GENERATORS.pdf