Simple AVR ISP Circuit

Here’s a design circuit for simplest AVR ISP circuit is shown here, with some notes. Ground and VCC must be connected to programmer, either to power programmer or to provide a reference voltage for lower voltage circuits. This is the figure of the circuit;

Most AVR microcontrollers use MOSI, MISO and SCK SPI pins for programming, but some such as ATmega128 use SCK and then UART TX and RX pins instead. The capacitor connected between Reset and Ground, and the resistor from Reset to Vcc should be fitted to give a slight delay to allow AVR to power up properly. These values are not critical, but if they are too large then the ISP programmer will need to be slowed down. The programming lines (SCK, MISO and MOSI) are best left just for programming, but if these pins must be used by the application, then resistors should be used to isolate the application circuitry, typically 4K7. This is especially important if TXD/RXD are used for programming, as UART chips tend to hold the lines. This should be fine for SPI or UART use or where pins are inputs but if you have to use these lines for higher current, then a multiplexer circuit may be needed, see STK200 schematics. Capacitors on the programming lines can cause problems, especially on SCK. If they have to be fitted, then they should be below 10nF and fitted as close as possible to AVR microcontroller pins. Some low cost programmers will have problems with even a 10nF capacitor on SCK or MOSI, so smaller is better.

Simple Electrification Circuit Unit

Here’s a design circuit that is intended for carrying out harmless experiments with high-voltage pulses and functions in a similar way as an electrified fence generator. The p.r.f. (pulse repetition frequency) is determined by the time constant of network R1-C3 in the feedback loop of op amp IC1a: with values as specified, it is about 0.5 Hz. The stage following the op amp, IC1b, converts the rectangular signal into narrow pulses. Differentiating network R2-C4, in conjunction with the switching threshold of the Schmitt trigger inputs of IC1b, determines the pulse period, which here is about 1.5 ms. The output of IC1b is linked directly to the gate of thyristor THR1, so that this device is triggered by the pulses. Here’s the figure of the schematic diagram;

The requisite high voltage is generated with the aid of a small mains transformer, whose secondary winding is here used as the primary. This winding, in conjunction with C2, forms a resonant circuit. Capacitor C3 is charged to the supply voltage (12 V) via R3.When a pulse output by IC1b triggers the thyristor, the capacitor is discharged via the secondary winding. The energy stored in the capacitor is, however, not lost, but is stored in the magnetic field produced by the transformer when current flows through it. When the capacitor is discharged, the current ceases, whereupon the magnetic field collapses. This induces a counter e.m.f. in the transformer winding which opposes the voltage earlier applied to the transformer. This means that the direction of the current remains the same. However, capacitor C2 is now charged in the opposite sense, so that the potential across it is negative. When the magnetic field of the transformer has returned the stored energy to the capacitor, the direction of the current reverses, and the negatively charged capacitor is discharged via D1 and the secondary winding of the transformer. As soon as the capacitor begins to be discharged, there is no current through the thyristor, which therefore switches off. When C2 is discharged further, diode D1 is reverse-biased, so that the current loop to the transformer is broken, whereupon the capacitor is charged to 12 V again via R3. At the next pulse from IC1b, this process repeats itself.

Tilt Sensor Alarm Circuit

This is a design for simple circuit of the tilt sensor alarm presented here can be fabricated using readily available inexpensive components. The circuit is a true transistor based design. Home made Tilt sensor for this circuit is an ordinary little glass/plastic bottle with two metal needles inserted through its cap, and a small quantity of water inside. A 9V alkaline battery is enough for powering the whole circuit. Here’s the figure of the schematic circuit;

In this figure shows that this circuit is based on two transistor as main control. There are BC 557 and BC 547. Usually, transistor T1 is in inactive state. When the sensor assembly is tilted, both needles inside the sensor (bottle) are short circuited by the water and a positive voltage is available at the base of T1 and it becomes active. Activation of T1 causes the activation of next transistors T2 and T3. After this, T2 supplies constant bias for T1 to make it latched and T3 triggers the SCR(T4) which in turn energizes the active piezo-sounder(BZ1). Once activated the circuit can be deactivated by depressing the power/reset switch S1. Preset pot P1 is deliberately added here to adjust the circuit sensitivity. This may become necessary if you are trying a different (readymade) tilt sensor. Similar, SCR(T4) and piezo sounder (BZ1) may be replaced with near equivalent parts. Resistor R3 (100-150 Ohm) is optional.

Simple Temperature Relay Circuit

This is a design circuit for temperature relay that can be used to signal a fire or set point for temperature monitoring function. You need to adjust P1 so that T1′s base voltage is 0.5V smaller than the emitter voltage at a temperature a little bit lower than the desired triggering (switching) temperature. This is the figure of the schematic circuit;

If the temperature increases then T1 and T2 start conducting and the relay is closed (ON). If you want to use it as a cold relay or to signal an inferior temperature limit, then Th1 and P1 change places. After the relay is triggering you need to open switch S1 in order to stop the circuit. The nominal value of P1 must be chosen according to the used NTC thermistor and the switching temperature to be adjusted.