These days, many embedded systems need a +5V power supply with special features like power-fail detection, zero-crossing signals for mains power supply, and the ability to keep built-in batteries charged. Here is the circuit for a power supply for embedded systems like this one. It has +5V, a charger for the battery, zero-crossing signals, and a power-fail signal. The circuit is made up of common, cheap parts. To get started right away, you just need to make a few simple adjustments with potmeter POT1. The circuit in Fig. 1 is made up of a step-down transformer (X1), two bridge rectifiers (BR1 and BR2), an adjustable voltage regulator (IC1), a 5V voltage regulator (IC2), a hex inverter Schmitt trigger (IC3), and a few discrete components. Connector CON1 is given 230V AC, 50Hz power from the mains. Fuse F1 keeps the input from getting too much power. Some of the noise from the mains power supply is blocked by resistor R1 and capacitor C1. Power transformer X1 has a secondary that is 12V and 2.5A. The standard 5V regulator IC2 is used to power CON3 with 5V and 1A. The 2.5A rectifying bridge is BR1. The main filtering cap (C4) should have a

value of at least 4700 F. IC2 gives out a steady +5V with a current of up to 1A. If you need more current, you can use regulators like the 78T05 (3A, 5V) or the 78S05 (2A, 5V). In real life, it is best to keep the load on 78XX to between 0.7 and 0.8A. The voltage from CON4's unregulated output ranges from 10V to 18V, depending on the transformer used and how much current is drawn from the power supply. This voltage isn't controlled, but it can be used for other things. The output is protected by the fuse F2. The power supply has a battery charger with a regulator IC1 that can be changed. Some parts of the system are powered by a 6V rechargeable battery when the mains power supply isn't working right. The potmeter POT1 is used to change the maximum voltage level across the rechargeable battery. Resistor R7 limits the amount of charging current that can flow. Regulator IC1 has an output voltage range of +1.25V to +8.2V, which can be changed with potmeter POT1. The regulators IC1 and IC2 are protected by diodes D1, D2, and D3. Bridge rectifier BR2 is only used to send signals at zero-crossing points. The value of capacitor C2 should be low. Its purpose is to cut only the very high frequency, not to filter the power from the mains. IC3 and the parameters of the pulses that are made can change the values of R2 and R3. When the voltage at test point TP2 drops below about 8V, CON5 sends a power-fail signal. Transistor T1 stops doing its job, and the control unit knows this because pin 3 of CON5 goes high. The power-fail signal's threshold voltage is controlled by zener diode ZD2 (7.5V) and resistors R4 and R5. Switching transistors are best for T1, but most npn silicon transistors with a high gain will also work. Pins 2, 3, and 4 of connector CON6 send out different signals at a rate that is twice as fast as the mains power supply (100 Hz). These signals are active near the zero crossings of the mains power supply and can be used for many things, such as: They can be used to measure the frequency of the mains power supply by the control unit. You can use these to make the control unit work at the same time as the zero crossings of the mains power supply. The strength of signal TP4 is related to the voltage on the transformer's secondary side. It can be measured by the control unit, which can then figure out

what X1's secondary voltage is. The outputs TP5 and TP3 work with either TTL or CMOS, depending on IC3. IC3 can be either CMOS or TTL. Some examples are 74HC14, 74HCT14, 74LS14, etc. It should have a Schmitt trigger built in.