QUASAR CODE # 3126 - SOUND ACTIVATED RELAY SWITCH
The Sound Activated Relay Switch is based on
a circuit published in Silicon Chip magazine,
9/1994, p31. The idea behind a VOX SWITCH
is that instead of the user pressing a switch to
activate a relay, the sound of the users voice
itself activates the relay. This gives hands-free
control over devices like tape recorders.
We have improved the original design by
adding an on-board potentiometer so you can
adjust the trigger sensitivity. The 3126v41
board also features:
1. Screw terminal blocks for all input and
output connections
2. The output is rated to switch up to 120Vac
or 24Vdc @ 1 Amp maximum current. The
relay itself may be rated to switch higher loads
but the PCB tracks limit the amount of current
and voltage that can be safely switched. The
output should NOT be used to switch mains
electricity directly but can instead be used to
switch a higher capacity off-board relay or
switch. The Common (C), Normally
Connected (NC), and Normally Open (NO)
connections are taken to a 3-pole terminal
block on the right hand edge of the board. For
more information please see our relay FAQ
sheet at
www.quasarelectronics.co.uk/pdf/relay-faq.pdf
3. MIC pin spacing increased to make the
MIC body stand off the board. This helps to
reduce a feedback loop that causes the relay to
not turn off (see Note 4 below)
4. “EXT MIC” terminal block input added.
This allows you to mount the supplied
microphone (MIC) away from the board for
convenience or if you are experiencing
feedback that causes the relay to continuously
reactivate or not turn off. Attach using
screened cable no more than 1 metre long. . If
you purchased the pre-assembled version you
will need to carefully unsolder the MIC from
the board first. NOTE: Only use an electret
type MIC as supplied
ASSEMBLY
Check of the components against the Listing.
Follow the overlay for component positions
and orientation. Mount the MIC about 3mm
above board. Note that if the relay is
triggering continuously then it may be
necessary to located the mic some distance
away from the board (like 0.5-1 metre) using
2-core screened cable (not supplied). This
problem is caused by vibration from the relay
itself retriggering the circuit.
Also note that C4 is located under the IC so the
IC socket has to be used. Make sure to get C3
(104) and C4 (101) in their correct places. If
you mix the 101 with the 104 the kit will not
work. Ensure diodes, electrolytic capacitors,
MIC, Q1 and IC1 are inserted with correct
orientation.
COMPONENTS
OPERATION
Audio is picked up by the microphone and fed
to the opamp IC1a. This is connected as a non-
inverting amplifier with a gain of 151 or
+43.6dB. The 100pF capacitor across the 150K
feedback resistor rolls off the high frequency
response above 10kHz to eliminate RF. The
output at pin 1 feeds two diodes, D2 and D3
Resistors 5%, 1/4W:
R1, R7 1K brown black red 2
R3 2K2 red red red 1
R2, R4, R8 10K brown black orange 3
R9 39K orange white orange 1
R5 1M brown black green 1
R6 200K (204) Koa
potentiometer
1
Capacitors
C4 100pF (101) monoblock 1
C3 0.1uF (104) monoblock 1
C2, C5 2u2 mini electrolytic cap 2
C1 10u mini electrolytic cap 1
Semiconductors
D1, D4 1N4004 2
D2, D3 1N4148 2
Q1 BC548 1
IC1 LM358 1
Miscellaneous
8 pin IC socket 1
RL1 Relay AZ-SH-112L (or
equivalent)
1
X1, X2 2 pole terminal block 2
X3 3 pole terminal block 1
L1 3mm LED
1
MIC Electret microphone
1
PCB # 3126v41 1
