Schema and assembly of the sound detector .

Presentation

The detector noise in question here is for Inrush a tape or a light bulb filament, when a sound picked up by a small microphone exceeds a predetermined noise level. The sensitivity range is very wide, the assembly is able to react to a spoken voice to five feet away from the microphone. Obviously, such sensitivity is not always desired, and the assembly has a setting to adapt it to your needs. I made this montage at the request of an officer of a resort, which wanted a "warning light passing noise" in a dining hall. History to set a limit not to exceed, with the children present at mealtime. Not easy, knowing that the game can also be to make the most noise to light the lamp ... The key is to set the rules precisely, is not it?

The diagram

While there are many circuits simpler than this, and who only a single transistor or a single integrated circuit as active. Both say immediately, I developed this pattern by having the desire to have something perfectly reproducible and that works well every time. I did not try to make it simple, because in this case will be simpler could be synonymous with trouble. In this scheme, no component is really critical, and take an approximate value for each of them should be no problem. The assembly consists of five distinct parts, I will describe separately: a microphone preamplifier, a rectifier diode, a comparator, a monostable (temporistateur), and a power output stage.

The microphone preamplifier

The preamp is based on the use of a transistor current. The differences lie in the unique addition of a resistor R6 in the emitter circuit of BC109 (Q1) to ensure good thermal stabilisaton, which guarantees stable operation even with large variations in ambient temperature. As this resistance is quite high (given its location) and it greatly reduces the gain of the amplifier stage, a 100 uF capacitor C2 is added to it in parallel. It behaves like an open circuit in the static and does not influence the bias DC voltage of the transistor, and behaves almost like a short circuit under dynamic conditions, ie when a signal is present in BF entry. This capacitor can therefore "recover" the gain lost due to the emitter resistor. The resistance R5 of 12 KO plays on the base bias of Q1 and also plays on the gain, which is here close to 20 dB .. If you believe this gain enough, you can increase the value of R5 220 to KO, value for which the gain will be approximately 32 dB, 12 dB or more. The microphone used is type electret capsule, chosen for its compactness, its high sensitivity and very low cost. If you prefer using a small microphone Dynamic 200 ohms, you can, but in this case, do not wire the components R1, R2 and C3, which are used here to supply the electret microphone. RC cell composed of R1 and C3 is absolutely necessary, it can isolate the power supply of the rest of the micro assembly. Without this decoupling unit, there would be high risk of coming into swing this floor preamp. Note that the power transistor Q1 is also undergoing a strong enough coupling, for the same reasons. The amplified signal appears at the collector of transistor Q1, and passes through a coupling capacitor, to prevent the DC voltage present on the collector to reach the potentiometer sensitivity adjustment that follows.

The rectifier diode

The rectifier diodes discussed here is based on the use of an integrated circuit type LM358 PDO, which amplifies the signal a little before straightening BF, ie to transform it into a DC voltage proportional to the amplitude of the signal picked up by the microphone. This section does not s'appuye the famous architecture in which the rectifier diode is inserted in the loop against the AO reaction to remove the conduction threshold of the diode. Here, no need for this device, we gladly leave to more elaborate arrangements. The signal to be processed is taken from the slider of the potentiometer RV1 used here span, and is applied to the inverting input of the AOP U1: A, first half of the LM358. The rate of amplification of the second floor is high, its value is determined by the ratio between the two resistors R10 and R9, which here is 100 (corresponding to 40 dB). If the sensitivity of the assembly seems too large in practical situations, feel free to lower the value of R10, up to 100K or less. Once the audio signal amplified again so it is applied to the recovery Patie itself, consisting of C9, C10, D1, D2 and R14. On the cathode of D1 (D1 in common, C10 and R14), there is a voltage that is proportional to the amplitude of the audio signal applied to the input of the rectifier (common R10 / C9). It only remains to compare this variable voltage to a fixed reference voltage, which will set the threshold for triggering the system.

The comparator

This is the easiest section of this assembly, it involves only three components U1: B (second half of the LM358), R15 and RV2. Exit 7 of the PDO U1: B is at high level (12V) when the voltage on the input 5 (+) is greater than the voltage at the input 6 (-). This same output is in contrast to the low state (0V) when the voltage on the input 5 (+) is less than the voltage at the input 6 (-). The rectified voltage applied to the input 5 (+) are all higher than the sound picked up is strong, there is indeed a time when this voltage exceeds the reference voltage (at terminal 6) and will switch out of AO from low to high. So you understand that there is a second adjustment will determine the trigger, and you will find a good compromise between the two settings offered by RV1 and RV2. Note that you can totally, as a simplification, replace the potentiometer RV2 by a fixed resistor of 1K to 4K7, and thus keep only the setting of RV1. Personally, I prefer to keep this pot RV2, which simplifies the settings in cases of extreme sensitivity.

The shot (timer)

This part was added to ensure a minimum temperature of outbreak, regardless of the time during which the sound signal picked up by the microphone has exceeded the threshold switching. The circuit is based on the use of a monostable type CD4528 or CD4538, which produces a pulse width well defined when applied to the input triggering
"Positive", a rising edge (logic that goes from low to high), which is precisely the case of the floor above when the sound picked up is hard enough. The activation time of the output (pulse width) is determined by the value of the components C11, R18 and RV3, and can be adjusted from just under one second and ten seconds. Note that this is the second half of the CD4528 (4538) was used, and only to facilitate routing of the IC. The first would moitiée quite agreed too! As it is not used, new outbreak and reset are connected to ground in order to remain fixed at a potential well and thus avoid "baguoter" from one state to another at the whim of the environment environment (it is still necessary - and we must get used - to connect to ground or more eating, all unused inputs of logic circuits).

The output stage power

This floor is essential to be able to drive loads other than a small low voltage lamp or LED. The use of a relay is here justified by the fact that the circuit can be controlled inductive (K7 tape recorder with a transformer for its power), which would be a problem if the output was made with a triac. The relay is controlled by an NPN transistor type universal type 2N2222. This transistor is protected by the diode D3 against surges caused by the relay during its deactivation. The relay should be selected according to the power consumed by the device to be controlled, contacts will be able to support the switched current. LED D4 serves as a control room, and is optional. To ensure that the relay contacts for a longer life, a cell RC series (C13 / R19) was added in parallel contacts, it avoids the production of sparks during switching.

Food

A single power supply is ample. This is what I have adopted, setting the output voltage to 12V.

The prototype:

Plate prototype testing:

Note that in the photo above is not n'apperçoit sections and monostable output stage, I did not need to experiment so they are already well-established ;-).

The printed circuit

PCB design finished.

PCB

Artwork

PCB is completed, it works.

See below the diode 1N4007 for the protection of the control transistor of the relay (diode), with its legs connected in heat sink to facilitate heat dissipation.

And below, the RC network to protect the relay contacts, eliminating the spark.

And canning

I opted for a plastic box Retex 4, which contains all the components and the power supply. The microphone is directed outward and is surrounded by rubber to absorb vibrations. The potentiometer settings are placed to the right of the box, the entry and exit control area are on the left. The power supply section (recovery, filtering, control) is performed "in air". I know, it's less beautiful than a printed circuit board.

Note: I had to surround the electromechanical relays in bubble wrap to absorb some noise that naturally occurs during switching. Without this, and with a setting of high sensitivity, the noise caused by the relay when it closed, is captured by the microphone ... that redéclanche all, ensuring an endless loop. This happens even when the sensitivity is at max, but difficult to do otherwise with this type of relay. To completely avoid this, one should be silent electromechanical relay (expensive) or a solid state relay, which was rejected here because of the type of load that it must be possible to connect the output. Fortunately, the merry band of children as "sound monitoring" should produce more noise than the relay, and there should be no need to push the detector's sensitivity too ...