Night Buzzer

If you want revenge that won’t kill someone, just make them go insane, have I got the thing for you!! I whipped this device up over the last couple of days and couldnt be more satisfied.
Place this in a room where somebody sleeps. While the light is on, this circuit will not make a peep. When the light turns off though, the circuit will stay quiet for about 10 seconds (determined by the 1000uf cap and the 22k resistor) and then turn on. When they hear it, they will get up and turn on the light to look for it, but of course it will turn off. Thinking that they are imagining things, they will turn off the light and go back to bed. 10 seconds later, they will hear it . . . it’s a vicious cycle.
How it works
When the LDR senses light, it turns Q1 on, which in turn makes Q2’s base more negative than positive: transistor (and buzzer) is off. When the LDR senses no light, it turns Q1 off. The capacitor discharges through the 22k resistor, all the while causing Q2’s base more negative than positive (the 10 sec delay). When it is done discharging, the 22k resistor biases the base of Q2 positively, which turns the transistor and the buzzer on. Simple as that.
To make a longer delay, just increase the capacitance. Since the 22k resistor serves a dual purpose: draining the capacitor and biasing Q2, it should not be made a higher value to increase the length of the delay because it then won’t be able to bias Q2 correctly. Of course you can experiment. I think that the value can be increased a bit more. If you want to shorten the delay, by all means, decrease the value of that resistor, just as long as the value is so low that Q2 will receive too much current through the base and damage it. I’m not sure if this will happen with so low a supply voltage, but beter 2 be safe than sorry.
The 5k POT and the 2.2k resistor in series with it work very well with the 3-16 volt buzzer. If it is a small room, lowering the value of the POT will make the buzzer quieter and harder to find. If you use a different buzzer, you will just have to experiment to find the right value of resistor(s). Just make sure that you turn the buzzer WAAAY down to where u can barely hear it, that way, they will have a very hard time finding it.
Any small small signal NPN transistor will work for NPNs in the circuit, i personally used 2n3904's cuz i have a ton of them, but like i said, anything will work.


This circuit is EXTREMELY sensitive to light. Just turning on a flashlight in the room will turn it off.


If requested, I will provide a PCB, although u will have to make sure that the capacitor, the 


POT, and the buzzer will fit on it.


Have fun . . . and don’t be too cruel.

AM Radio Receiver (4 Cricket Live Comment)

     

      

            Here's a very simple AM Radio circuit I've designed couple of years ago. Don't know whether anybody listen to AM stations anymore. But I still use it (maybe I wanted to listen to my own built radio lol..).

               The radio section is wired using a single transistor(BF494) and it was so amazing that an audible sound is recovered at the output which is faint though. It doesn't use any external antenna and the sensitivity/selectivity of the receiver is pretty good. However I used an amplifier(TA 7368P Toshiba, Low voltage) which drives an 8ohm/1W 4" speaker inside a box rocks the entire room with a high fidelity audio that is unbelievable and outperforms Super heterodyne ones in this regard . 

            

           It is a reflex receiver.The audio recovered at inductor L is rather strong comparing to ZN414 and free from oscillations.(I've never succeeded in building ZN414 which always give me annoying motor boating and chirping crappy I say!)Using a flat ferrite bar antenna allows local reception for a pocket radio and a big rod antenna captures stations beyond 200miles! So it doesn't require an external wire antenna, adding it only helps in electrical noise catch.Only critical part in the circuit is inductor L, its optimum value gives excellent results. Make rf parts close to the transistor. I made it on a 1.5" ultra small pcb. 2 x AA battery lasts very long.

          

           Another important thing is that the radio is absolutely silent in between the stations - means no noise at all if no any electrical interference which is a plus point over super heterodyne receivers. It was so amazing to tune it during power failure period. So I'll call it a true radio

Strobe Light

Strobe lights are widely used by disco lovers to create wonderful visual effects in disco halls and auditoria. The circuit of a battery operated portable miniature strobe light, which can be constructed using readily available inexpensive components, is described here. For convenience and simplicity, an ordinary neon lamp is used here in place of the conventional Xenon tube. The whole gadget can thus be easily accommodated in a small cabinet, such as a mains adaptor cover, with a suitable reflector for neon lamp to give a proper look. Since current requirement of this circuit is very small, it may be powered by two medium-size dry cells (3V) or Ni-Cd cells (2.4V). Transistors T1 and T2 in the circuit form a complimentary-pair amplifier. When switch S1 is momentarily depressed, the circuit oscillates because of the positive feedback provided via resistor R2 and capacitor C1 to the base of transistor T1. The sharp pulses in the secondary winding induce a high voltage in primary winding of transformer X1, which in fact is a line driver transformer (used in reverse) which is generally used in 36cm TV sets. High voltage pulses induced in primary side are rectified by diode D1 and rapidly charge reservoir capacitor C2 to nearly 300V DC. When switch S1 is released, capacitor C2 holds the voltage level for a finite period while capacitor C3 charges slowly through resistor R3. When voltage across capacitor C3 becomes high enough, neon strikes and the capacitor rapidly discharges through the lamp. When voltage across capacitor C3 falls below the extinguishing potential of neon lamp, it goes off and capacitor C3 starts charging again. This cycle keeps on repeating for a short time, based on the reservoir capacitor C2’s value. Precautions. The neon lamp flasher section of this circuit carries dangerously high voltages. All precautions should therefore be taken for protection. Before any repair work, discharge capacitor C2 using a short length of wire with a 100k resistor connected in series. 

TV Remote Blocker

The 555 IC is wired as an astable multivibrator for a frequency of nearly 38 kHz. This is the frequency at which most of the modern TVs receive the IR beam. The transistor acts as a current source supplying roughly 25mA to the infra red LEDs. To increase the range of the circuit simply decrease the value of the 180 ohm resistor to not less than 100 ohm.
It is required to adjust the 10K potentiometer while pointing the device at your TV to block the IR rays from the remote. This can be done by trial and error until the remote no longer responds.

Remote-operated Master Switch

 

 Generally, a bedside master switch is used to switch on lamps both indoors and outdoors when there is a threat of intruder. This circuit can be used to activate the master switch from the bed without searching for the switch in darkness. It can be activated by the TV remote handset. The security lamps glow for three minutes and then turn off. The circuit is sensitive and can be activated from a distance of up to 25 meters. 

Apparatus Needed to prepare Remote-operated Master Switch Bridge rectifier

·                     Zener iode

·                     Relay

·                     Transistor /SCR

·                     Resistor and Capacitors

·                     IC CA 3130

·                     3way switch

           IR receiver module TSOP 1738 (IRX1) is used to sense the pulsed 38kHz IR rays from the TV remote handset. The IR receiver module has a PIN photodiode and a preamplifier enclosed in an IR filter epoxy case. Its open-collector output is 5 volts at 5mA current in the standby mode. In the standby mode, no IR rays from the remote handset fall on the IR receiver, so its output pin 3 remains high and LED1 doesn’t glow. Through resistor R2, the base of transistor T1 remains high and it does not conduct. As a result, the voltage at pin 3 of IC CA3130 (IC1) remains low.
         The potential divider comprising resistors R4 and R5 maintains half of 5.1V at pin 2 of IC1. In brief, the voltage at pin 2 of IC1 is higher than at pin 3 and its output remains low. LED2 remains ‘off’ and transistor T2 does not conduct. Relay RL1 remains de-energised and, as a result, security lamps (both indoors and
outdoors) remain switched off.

           When you press any key of the remote TV handset, IR rays fall on the receiver (IRX1) and its output goes low. LED1 flashes in sync with pulsation of the IR rays. At the same time, transistor T1 (BC558) conducts to take pin 3 of IC1 high. IC1 is used as a comparator with timer action.

            When transistor T1 conducts, pin 3 of IC1 gets a higher voltage than pin 2 making the output of IC1 high. Meanwhile, capacitor C4 charges to full voltage and keeps pin 3 high for a few minutes even after T1 is non-conducting. Resistor R3 provides discharge path for capacitor C4, which decides the time period for which the output of comparator IC1 should remain high. The high output of IC1 energises relay RL1 through relay-driver transistor T2. Thus the load, i.e., security lamps, turn on for three to four minutes. LED2 glows to indicate activation of the relay as well as switching ‘on’ of the security lights. Connect a single-pole, single-throw ‘on’/‘off’ switch (MS) to activate the security lamps manually when required.
          Zener diode ZD1 provides 5.1V DC for safe operation of the IR receiver and associated circuit. Power for the circuit is derived from a step-down transformer (X1) and a bridge rectifier comprising diodes D1 through D4. Smoothing capacitor C1 removes ripples, if any, from the power supply. Assemble the circuit on a general purpose PCB and enclose in a suitable cabinet. Drill holes on the front panel for mounting the IR sensor and LEDs. Connect the master switch between the normally-open (N/O) contact and pole of relay RL1 so that the master switch can be used when needed. The relay contacts rating should be more than 4A. Mount the unit near the master switch using minimal wiring

Electronic Candles

          Here is a simple circuit that can produce the effect of candle light in a normal electric bulb. A candle light, as we all know,resembles a randomly flickering light. So, the objective of this project activity is to produce a randomly flickering light effect in an electric bulb.

           To achieve this, the entire circuit can be divided into three parts. The first part comprises IC1 (555), IC2 (74LS164), IC3 (74LS86), IC4 (74LS00) and the associated components. These generate a randomly changing train of pulses. The second part of the circuit consists of SCR1 (C106), an electric bulb connected between anode of SCR1 and mains live wire, and gate trigger circuit components. It is basically half-wave AC power being supplied to the electric bulb. The third part is the power supply circuit to generate regulated 5V DC from 230V AC for random signal generator. It comprises a step down transformer (X1), full-wave rectifier (diodes D3 and D4), filter capacitor (C9), followed by a regulator (IC5). The random signal generator of the circuit is built around an 8-bit serial in/parallel out shift register (IC2). Different outputs of the shift register IC pass through a set of logic gates (N1 through N5) and final output appearing at pin 6 of gate N5 is fed back to the inputs of pins 1 and 2 of IC2. The clock signal appears at pin 8 of IC2, which is clocked by an astable multivibrator configured around timer (IC1). The clock frequency can be set using preset VR1 and VR2. It can be set around 100 Hz to provide better flickering effect in the bulb.
           The random signal triggers the gate of SCR1. The electric bulb gets AC power only for the period for which SCR1 is fired. SCR1 is fired only during the positive half cycles. Conduction of SCR1 depends upon the gate triggering pin 3 of IC2, which is random. Thus, we see a flickering effect in the light output. Assemble the circuit on a general purpose PCB and enclose it in a suitable case. Fix bulb and neon bulb on the front side of the cabinet. Also, connect a power cable for giving AC mains supply to the circuit for operation. The circuit is ready to use. Warning. Since the circuit uses 230V AC, care must be taken to avoid electric shock

fRIENDLY CHARGER FOR MOBILE PHONES

Most mobile chargers do not have current/voltage regulation or short-circuit protection. These chargers provide raw 6-12V DC for charging the battery pack. Most of the mobile phone battery packs have a rating of 3.6V, 650mAh.  For increasing the life of the battery, slow charging at low current is advisable. Six to ten hours of charging at 150-200mA current is a suitable option. This will prevent heating up of the battery and extend its life. 

   The circuit described here provides around 180mA current at 5.6V and protects the mobile phone from unexpected voltage fluctuations that develop on the mains line. So the charger can be left ‘on’ over night to replenish the battery charge. The circuit protects the mobile phone as well as the charger by immediately disconnecting the output when it senses a voltage surge or a short circuit in the battery pack or connector. It can be called a ‘middle man’ between the existing charger and the mobile phone. It has features like voltage and current regulation, over-current protection, and high- and low-voltage cut-off. An added speciality of the circuit is that it incorporates a short delay of ten seconds to switch on when mains resumes following a power failure. This protects the mobile phone from instant voltage spikes.
   The circuit is designed for use in conjunction with a 12V, 500mA adaptor (battery eliminator). Op-amp IC CA3130 is used as a voltage comparator.
It is a BiMOS operational amplifier with MOSFET input and CMOS output. Inbuilt gate-protected p-channel MOSFETs are used in the input to provide very high input impedance. The output voltage can swing to either positive or negative (here, ground) side. The inverting input (pin 2) of IC1 is provided with a variable voltage obtained through the wiper of potmeter VR1. The non-inverting input (pin 3) of IC1 is connected to 12V stabilised DC voltage developed across Zener ZD1. This makes the output of IC1 high.
       After a power resumption, capacitor C1 provides delay of a few seconds to charge to a potential higher than of inverting pin 2 of CA3130, thus the output of IC1 goes high only after the delay. In the case of a heavy power line surge, zener diode ZD1 (12V, 1W) will breakdown and short pin 3 of IC1 to ground and the output of IC1 drops to ground level. The output of IC1 is fed to the base of npn Darlington transistor BD677 (T2) for charging the battery. Transistor T2conducts only when the output of IC1 is high. During conduction the emitter voltage of T2 is around 10V, which passes  through R6 to restrict the charging current to around 180 mA.Zener diode ZD2 regulates the charging voltage to around 5.6V.

When a short-circuit occurs at the battery terminal, resistor R8 senses the over-current, allowing transistor T1 to conduct and light up LED1. Glowing of LED2 indicates the charging mode, while LED1 indicates short circuit or over-current status. The value of resistor R8 is important to get the desired current level to operate the cut-off. With the given value of R8 (3.3 ohms), it is 350 mA. Charging current can also be changed by increasing or decreasing the value of R7 using the ‘I=V/R’ rule. Construct the circuit on a common PCB and house in a small plastic case.Connect the circuit between the output lines of the charger and the input pins of the mobile phone with correct polarity.