Additionally, multiple touch plates can be attached to the control line at various points. The
control line can be up to 100 feet in total length.
Q1 acts as an emitter follower and as an in-phase amplifier to cancel long line capacitances for
the shielded control line. With in-phase feedback upon the control line, uniform sensitivity is
achieved whether the shielded control line is short or long.
Q1 should be selected so that 1.2 to 1.4 volts appear at the emitter. This requires a fairly high
gain transistor. Or, changing the value of the 6.8M resistor between Q1's base and collector will
also effect the 1.2 to 1.4 volts at the emitter. Any general purpose, high gain transistor can be
used for Q1.
A neon bulb protects Q1 from static discharge.
The signal is coupled to Q2. Q2 can be a general purpose transistor. Here, Q2 goes into
conduction when a touch plate is touched and signal is generated at the base of Q1. A .1 uf.
capacitor is quickly discharged at Q2's collector through the 15K resistor. Recharge of this
capacitor comes through the 2.7M resistor to the +12V line.... a much longer recharge time
constant. This arrangement assures a "debouncing" of the 60 Hz signal resulting from a touch on
a touch plate.
A1 (pins 1,2,3) (a Schmidt trigger) sharpens up the DC pulse generated by Q2. A single 15
millisecond pulse is generated as a result of the time constant formed by the .01 uf. capacitor and
1.5M resistor.
This 15 ms. Pulse is routed either through A1 (pins 4,5,6) or A1 (pins 8,9,10). Whichever
amplifier routes the pulse is determined by the contact position of the magnetically biased
bistable relay. If the relay contacts are open, then A1 (pins 4,5,6) passes the signal. Otherwise it
is passed by A1 (pins 3,9,10).
A1 (pins 11,12,13) form an inverting amplifier which is part of the pulse routing circuit. The
time constant formed by the 4.7M and .047 uf. capacitor is to slow response of the routing during
transitions of on and off by the relay.
Q3 and Q4 are emitter follower amplifiers. When Q3 conducts, a negative pulse is routed
through the relay via "B" with a return path from the relay through "C," then through the 330
ohm resistor connected to the +12 volts located at the base of Q4 (which at this moment is not
conducting). Similarly, when routed through Q4 (Q4 conducting), the reverse is true -- a
negative pulse is routed through "C."
This arrangement, then, gives the relay coil a "push-pull" voltage effect depending upon the
voltage polarity routing of the 15 millisecond pulse.
The relay is a 5-12 volt reed relay with a coil resistance between 300 and 400 ohms. Normally
such a relay must have current through the coil in order to close and hold the contacts. In this
case, we fashion this relay to our needs, converting it to a bistable relay.... a relay that needs only
a short pulse to alternately open and close the contacts. (For example, a positive pulse to close
the contacts, a negative pulse to open the contacts)
Using a small circular ceramic magnet (I used a 3/8" diameter by 1/8" deep hobby magnet), one
can position the magnet in such a way that the relay becomes bistable. When positioning the
magnet, one must observe magnetic polarity since the wrong magnetic polarity will bias the relay
in such a way that the unit will not respond. When the polarity of the magnet is correct -- and
the position of the magnet with respect to the the coil is proper, touches at the touch plates will
alternately toggle the relay from on to off with each touch of a touch plate. Important note: There must be a load plugged into the A.C. receptacle.
Move the magnet horizontally along the coil length experimentally to find the ideal location for
bistable relay operation. Once a position is found, silicon rubber sealer will secure the magnet
into position.
The relay contacts are "amplified" by use of a power triac such as an RCA 40429 or T2710. Any
general purpose triac usually works here. Such amplification routes the heavy current load away
from the reed relay contacts.
The power supply is a simple single diode supply which is "centered" between the hot and
ground side of the AC line. This reduces a shock hazard should someone accidentally come in
contact with the shielded control line and ground. A 68K resistor isolates both the hot and
ground AC line contacts. A 12 volt zener diode protects the voltage from exceeding 12 volts.
Each touch plate connected to the shielded control line must be isolated with a 10 megohm
resistor. This assures no shock hazard for persons touching the touch plates. Any number of
touch plates can be attached to the center line of the control line. However some caution should
be used to make sure these touch plates are not so big and mounted in such a way that they get
"grounded" out reducing circuit sensitivity. Touch plates, then, must be mounted on insulated
surfaces.
NOTE: It is VERY important that the ground and +12 volts lines are routed exactly as drawn in
the schematic diagram. Violation of this note can result in the circuit becoming unstable with
power line borne interference (such as the turning on and off of a motor or other inductive load in
the nearby vicinity).
Any shielded wire can serve as a control line.
Page Modified September 29,2001