Simple Acoustic Sensor

This acoustic sensor was originally developed for an industrial application (monitoring a siren), but will also find many domestic applications. Note that the sensor is designed with safety of operation as the top priority: this means that if it fails then in the worst-case scenario it will not itself generate a false indication that a sound is detected. Also, the sensor connections are protected against polarity reversal and short-circuits. The supply voltage of 24 V is suitable for industrial use, and the output of the sensor swings over the supply voltage range.

Circuit diagram :

Simple Acoustic Sensor Circuit Diagram

The circuit consists of an electret micro-phone, an amplifier, attenuator, rectifier and a switching stage. MIC1 is supplied with a current of 1 mA by R9. T1 amplifies the signal, decoupled from the supply by C1, to about 1 Vpp. R7 sets the collector current of T1 to a maximum of 0.5 mA. The operating point is set by feedback resistor R8. The sensitivity of the circuit can be adjusted using potentiometer P1 so that it does not respond to ambient noise levels. Diodes D1 and D2 recitfy the signal and C4 provides smoothing. As soon as the voltage across C4 rises above 0.5 V, T2 turns on and the LED connected to the collector of the transistor lights. T3 inverts this signal.

If the microphone receives no sound, T3 turns on and the output will be at ground. If a signal is detected, T3 turns off and the output is pulled to +24 V by R4 and R5. In order to allow for an output current of 10 mA, T3’s collector resistor needs to be 2.4 kΩ. If 0.25 W resistors are to be used, then to be on the safe side this should be made up of two 4.7 kΩ resistors wired in parallel. Diode D4 protects the circuit from reverse polarity connection, and D3 protects the output from damage if it is inadvertently connected to the supply. 

On/Off Infrared Remote Control

Therefore,
the first one you find allows you to turn off the light and enjoy your
movie in the best possible conditions. The infrared receiver part of
our project is entrusted to an integrated receiver (Sony SBX 1620-52)
which has the advantage of costing less than the components required to
make the same function. After being inverted by T1, the pulses
delivered by this receiver trigger IC2a, which is nothing other than a D
flip-flop configured in monostable mode by feeding back its output Q
on its reset input via R4 and C3. The pulse that is produced on the
output Q of IC.2A makes IC.2B change state, which has the effect of
turning on or turning off the LED contained in IC3.

This
circuit is an opto triac with zero-crossing detection which allows our
setup to accomplish switching without noise. It actually triggers the
triac T2 in the anode where the load to be controlled is found. The
selected model allows us to switch up to 3 amperes but nothing should
stop you from using a more powerful triac if this model turns out to be
insufficient for your use. In order to reduce its size and total cost,
the circuit is powered directly from the mains using capacitor C5 which
must be a class X or X2 model rated at 230 volts AC.

This type
of capacitor, called ‘self-healing’, is the only type we should use
today for power supplies that are connected to ground. ‘Traditional’
capacitors, rated at 400 volts, do not really have sufficient safety
guarantees in this area. Considering the fact that the setup is
connected directly to the mains, it must be mounted in a completely
insulated housing. A power outlet model works very well and can easily
be used to inter-space between the grounded wall outlet and that of the
remote control device.

Based on this principle, this setup
reacts to any infrared signal and, as we said before, this makes it
compatible with any remote control. On the other hand, it has a small
disadvantage which is that sometimes it might react to the ‘normal’
utilization of one of these, which could be undesirable. To avoid that,
we advise you to mask the infrared receiver window as much as possible
so that it is necessary to point the remote control in its direction
in order to activate it.

Diagram Radio Band Position Display

This circuit is an add-on unit for radio receivers that lack band-position display. The circuit presented here can show up to nine bands. It also incorporates a novel feature to make the display dance (blink) with the audio level from the receiver. The power-supply for the circuit can also be derived from the radio-set. The conversion of selected channel to BCD format is achieved using diodes D1 through D15 in con-junction with resistors R4 to R7. The voltages developed across these resistors (R4 through R7) serve as logic in-puts to BCD inputs of BCD to 7-segment de-coder IC1 (CD4511).

Circuit diagram :

Radio Band Position Display Circuit Diagram

When all switches are in  ‘off’ state, the volt-age across resistors R4 through R7 is logic zero, but when any of the switches S1 through S9 is slided to  ‘on’ position, the output across these resistors changes to output proper BCD code to represent the selected channel. This BCD code is converted to 7-segment display by IC1. By this arrangement of diodes, the need for another decimal-to-BCD converter IC and associated parts is obviated. Switches S1 through S9 are actually parts of existing band-switch of the radio. 

Usually, one or two changeover contacts would be found extra in the modular pushbutton-type band-switches of the radios. IC1’s display blanking pin 4 is connected to a display-blinker-control circuit wired around transistors T1 and T2. A small part of the audio signal from the speaker terminals is applied to rectifier diode D16 and filter capacitor C1 to pro-duce a pulsating DC across preset VR1. The sliding contact of preset VR1 is connected to the base of emitter-follower stage comprising transistor T2. The out-put of transistor T2, as amplified by transistor T1, is connected to pin 4 of IC1.Thus turning  ‘on’/‘off’ of display is con-trolled by the pulsating voltage developed from audio output of radio.

The power-supply regulator stage is needed only when radio power-supply is greater than 6V DC.

Author : M.K. Chandra Mouleeswaran Copyright : Electronic for you 2000