Wire loop alarm

Thread Starter

Kevil

Joined Jun 28, 2020
134
I would like to build the wire loop alarm.

1. If I understand it correctly at 4.8MΩ when I add both resistors there is insufficient voltage in the gate of the mosfet and therefore the connection between drain and source cannot be made.
2. At 4.7MΩ (wire break) Is the voltage across the resistor already less and a connection between drain and source will occur?
3. What exactly is the purpose of the capacitor in this circuit?

Mainly I am trying to understand how best to calculate the other components "1x mosfet", "2x resistors" and "1x capacitor" to the 9V battery.

TDS for 2N7000 is here
 

bertus

Joined Apr 5, 2008
21,897
Hello,

I have copied the schematic:
wire-loop-alarm-circuit.png
R1 and R2 make a voltage divider when the wire is placed.
The voltage at the gate is low and the mosfet will be inactive.
When the wire breaks, the capacitor will be charged and the gate voltage will rise above the threshold voltage and activate the mosfet, so the buzzer will sound.
The capacitor will give a slight delay and stabalize the voltage at the gate when the wire is placed.

Bertus
 

MisterBill2

Joined Jan 23, 2018
11,887
In the circuit shown, why is R2, 100K ohms, even required?
Why not hold the voltage between gate and source at zero? And then when the wire is broken the voltage between gate and source will rise to the battery voltage, which will drop a bit as the load starts drawing current.
With the transistor totally cut off the standby current will be the minimum possible.
 

Thread Starter

Kevil

Joined Jun 28, 2020
134
How to modify the circuit to replace MOSFET by Infineon BTS3104SDL (104 mOhm single channel smart low side power switch for 12V & 24V Application) or simmilar power switch?
 

MisterBill2

Joined Jan 23, 2018
11,887
But when the loop is open you have an antenna connected to the mosfet gate.
Consider the reactance of that 0.1 microfarad capacitor at any Radio Frequency. The small voltage developed at the gate will be difficult to measure, and certainly to small to cause any conduction. This is not going to be a large loop high above the ground, at least I don't think it is planned to be.
 

ericgibbs

Joined Jan 29, 2010
15,526
Hi Salts,
IF the length of the wire loop was long, say around a field/garden the capacitance to ground could be high, so any atmospheric electrical discharge could drive an unlimited charge into the MOSFET Gate junction and take it out.
The 100k series resistor should help reduce this Gate charge current.

E
 

MisterBill2

Joined Jan 23, 2018
11,887
Please explain what a disconnected 100k resistor (when loop is broken) will do. I'm having trouble understanding your argument.
In addition to the 100K resistor not being in the circuit when the loop is opened. at that point the intention is to have the transistor switched on, into full saturation, to power the alarm sounder. Under that condition it does not seem that any believable amount of noise on top of the DC bias will cause a problem.
A possible exception would be if the whole system were close to a high power transmitting antenna. Going back to post #1, I see that we do not have any information as to where the lop might be set up..
Typically wire break loops are used by campers to serve as alerts for animal intrusions. That is what I have read previously. If one has a few seconds of advance warning it is possible to grab one's shotgun and discourage a bear with a few rounds at close range.
 

MrSalts

Joined Apr 2, 2020
1,762
Hi Salts,
IF the length of the wire loop was long, say around a field/garden the capacitance to ground could be high, so any atmospheric electrical discharge could drive an unlimited charge into the MOSFET Gate junction and take it out.
The 100k series resistor should help reduce this Gate charge current.

E
Do you expect the antenna effect to occur just during the first 4-seconds as the capacitor is charged by the 4.7M resistor and one RC time-constant (4.7 seconds) charges the gate (and C1) over 6 volts? Or do you expect to keep hearing some sort of interference or transmitted music/voice as if it is a DC signal countering the battery?

In either case, what result do you expect from an antenna that is essentially a shorted to ground through C1 (at radio frequencies)? Do you expect to hear a detectable warble in the buzzer? AM or FM Modulated voices or music coming through the buzzer? Or do you expect the RF to pull the gate voltage low in a DC to make the buzzer unreliable in the on-state?
 
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