Decibel Meter

The circuit below responds to sound pressure levels from about 60 to 70 dB. The sound is picked up by an 8 ohm speaker, amplified by a transistor stage and one LM324 op-amp section. You can also use a dynamic microphone but I found the speaker was more sensitive. The remaining 3 sections of the LM324 quad op-amp are used as voltage comparators and drive 3 indicator LEDs or incandescents which are spaced about 3dB apart. An additional transistor is needed for incandescent lights as shown with the lower lamp. I used 12 volt, 50mA lamps. Each light represents about a 3dB change in sound level so that when all 3 lights are on, the sound level is about 4 times greater than the level needed to light one lamp. The sensitivity can be adjusted with the 500K pot so that one lamp comes on with a reference sound level. The other two lamps will then indicate about a 2X and 4X increase in volume.

In operation, with no input, the DC voltage at pins 1,2 and 3 of the op-amp will be about 4 volts, and the voltage on the (+) inputs to the 3 comparators (pins 5,10,12) will be about a half volt less due to the 1N914 diode drop. The voltage on the (-) comparator inputs will be around 5.1 and 6.5 which is set by the 560 and 750 ohm resistors.
When an audio signal is present, the 10uF capacitor connected to the diode will charge toward the peak audio level at the op-amp output at pin 1. As the volume increases, the DC voltage on the capacitor and also (+) comparator inputs will increase and the lamp will turn on when the (+) input goes above the (-) input. As the volume decreases, the capacitor discharges through the parallel 100K resistor and the lamps go out. You can change the response time with a larger or smaller capacitor.
This circuit requires a well filtered power source, it will respond to very small changes in supply voltage, so you probably will need a large filter capacitor connected directly to the 330 ohm resistor. I managed to get it to work with an unregulated wall transformer power source, but I had to use 4700uF. It worked well on a regulated supply with only 1000uF.

Audio Booster

The 2N3392 transistor is a low-noise type in a TO-92 housing and can be replaced by a NTE199 or ECG199. Potentiometer R5 of 100K is a linear type with an on/off switch attached. The value of C1 may need to be between 0.05μF and 0.1μF (47nF/100nF). Experiment with the value for best performance.

Game Show Timer

Here is a really simple game show timer designed with the beginner in

mind! The power source is an ordinary 12 volt lantern battery or battery

pack made up of C or D cells. The lamps may be ordinary flashlight bulbs;

the prototype uses 750 mA Krypton types (KPR112) with wires soldered

directly onto the bulbs. The control devices may be just about any nonsensitive

gate SCRs or triacs with a current rating of a few amps. SCRs are

shown and triacs may be substituted by connecting MT1 to the negative

battery terminal and MT2 to the lamps. In fact, the two types of devices

could probably be mixed if that is what the junk box has to offer! The

zener is an ordinary 9 volt, 1 watt type like the 1N4739. The 100 ohm resistor

may be any type with a rating of 1/8 watt or more. The reset/power

button is an ordinary toggle switch and the pushbuttons are any

convenient normally-open type.

The entire circuit may be built into one box with two wires running out to

each contestant's pushbutton switch or the SCR, lamp, and switch may be

in a remote box for each contestant with three wires running back to the

base unit. Many more than three contestants may be added by repeating

the pattern.

Car Back Up Alarm

The brake lights of the automobile trigger this circuit on and off. This save the annoyance of the alarm when it is not needed.

Mobile Phone Battery Charger

Mobile phone chargers available in the market are quite expensive.The circuit presented here comes

as a low-cost alternative to charge mobile telephones/battery packs with a rating of 7.2 volts, such as Nokia 6110/6150.

The 220-240V AC mains supply is downconverted to 9V AC by transformer X1. The transformer output is rectified by diodes D1 through D4 wired

in bridge configuration and the positive DC supply is directly connected to the charger’s output contact, while the negative terminal is connected through current limiting

resistor, R2.

LED2 works as a power indicator with resistor R1 serving as the current limiter and LED3 indicates the charging status.During the charging period, about 3 volts

drop occurs across resistor R2, which turns on LED3 through resistor R3.

An external DC supply source (for instance, from a vehicle battery) can also be used to energise the charger, where resistor R4, after polarity protection diode D5, limits the input current to a safe value.The 3-terminal positive voltage regulator LM7806 (IC1) provides a constant voltage output of 7.8V DC since LED1 connected between the common terminal (pin 2) and ground rail of IC1 raises the output voltage to 7.8V DC. LED1 also serves as a power indicator for the external DC supply.

After constructing the circuit on a veroboard, enclose it in a suitable cabinet.A small heat sink is recommended for IC1.

Song Number Display

Here’s a circuit to display the song number in an audio system for quick reference to songs. It also serves the purpose of an extra visual indicator in modern audio systems

When the power is switched on, the power-on-reset circuit comprising 3.3k resistor R20 and 1μF, 25V capacitor C6 resets the counters, showing ‘00’ in the display. One can also reset the display to zero at any time by pressing reset switch, S1.
When the first song starts playing, the output pins of IC1 (KA2281) go low and capacitor C5 starts charging. This forward biases transistor T1 and hence the input to IC3 at pin 1 goes to high state. As a result, the output of the counter goes to the next state, showing 01 on the display. The counter remains in this state until the song is completed.
During the time gap before the next song starts playing, capacitor C5 discharges. After discharging of capacitor C5, the input to IC3 becomes low again. When the song starts, 02. You can adjust VR3 to change the time gap setting. This must be set such that the circuit doesn’t respond to short gaps, if any, within a song and responds only to long gaps between different songs.
Transistor T2 helps in gap-delay adjustment. The intensity of LED11 diminishes when a song is completed and the counter is ready to accept the next pulse.
Connect the input to the preamp output or equaliser output of the audio system. Adjust VR1 and VR2 to get the correct audio-level indication. If you are already using KA2281 for audio-level indication, just connect diodes D1 and D2 as shown in this circuit.
Note that the counter counts the songs by detecting the gaps. Therefore any long gap within a song may cause false triggering and the display will also be incremented. However, as this is very unlikely to happen, the circuit shows the correct song number almost all the time.

Touch Switch

A touch switch is a switch that is turned on and off by touching a wire contact, instead of flicking a lever like a regular switch. Touch switches have no mechanical parts to wear out, so they last a lot longer than regular switches. Touch switches can be used in places where regular switches would not last, such as wet or very dusty areas.

Parts List:

C1 - 10uF 16V Electrolytic Capacitor

R1, R2 2 - 100K 1/4 Watt Resistor

R3 - 10 Meg 1/4 Watt Resistor

U1 - 4011 CMOS NAND Gate IC

MISC - Board, Wire, Socket For U1

Notes:

1. The contacts can be made with just two loops of wire close together, or two squares etched close together on a PC board.

2. When activated, the output of the circuit goes high for about one second. This pulse can be used to drive a relay, transistor, other logic, etc.

3. You can vary the length of the output pulse by using a smaller or larger capacito for C1.