Circuits designed by David Johnson, P.E.
Last Updated on:
Monday, December 25, 2017 02:09 PM
List of Dave's Circuit Designs
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Test Monitors, Water
Water Seepage Monitor
A while back I got a
request from a company who needed a water seepage detector. They wanted a device
that ran on 24vac and detected water using a fully isolated circuit. They wanted
a circuit which switched off 24vac to a load when water was detected. The circuit
below performs this function. By changing the phototransistor’s position in the
detector circuit, the circuit logic can be changed so the 24vac source can be turned
on when water was detected. This might be handy to activate a pump.
|The sensor is made
using two stainless steel probes. The probes would be positioned as needed, so the
water would come in contact with them when the water reached a certain level. Since
the detection circuit is isolated, the two wires leading to the probe could be located
some distance away from the rest of the circuitry.
|A 15v supply is
developed using two capacitors connected to the 24vac power source. The circuit is
referenced to the two sources of the AC switching transistor Q1 and Q2. The AC is
first rectified and then filtered. A simple zener diode limits the DC voltage to
15v. A Schmitt trigger is used to form a 100KHz oscillator circuit which drives the
primary of a small transformer. A ferret bead with a 0.25 inch diameter forms the
transformer core.20 turns are used on the primary and 10 turns on the secondary.
The secondary winding is connected to an optoisolator and to the two water detection
probes, through a resistor. The optoisolator has two infrared LEDs inside and a
phototransistor. When the two probes come in contact with water current flows in
both directions through the isolator’s two LEDs. Light from those LEDs is detected
by the internal phototransistor. The output of the phototransistor is connected to a
second Schmitt trigger, which drives two n-channel FETs. The transistors are wired
in such a way that when turned on they conduct current in both directions. The
transistors can thus switch 24vac power to the load. Using the transistor shown, the
current should not exceed 10 Amps.
on Drawing Below to view PDF version of Schematic