Constant-current controller for LED light string

1. A constant-current controller for a LED light string having multiple lighting units, the constant-current controller comprising:

an operating voltage generating circuit for converting an AC voltage to an operating voltage;

a control unit connected to the operating unit to receive the operating voltage, storing a control program and having multiple output terminals;

multiple constant current circuits connected respectively between the AC voltage and multiple second switches, and converting the AC voltage to a DC voltage;

the multiple second switches adapted to connect to the lighting units of the LED light string respectively;

multiple first switches respectively connected to the output terminals of the control unit and the second switches;

the control unit outputting driving signals according to the control program to turn on the first switches, and the second switches being subsequently turned on to allow electric currents from the constant current circuits to flow through the second switches thus activating the lighting units of the LED light string.

2. The constant-current controller as claimed in claim 1 , wherein

each of the first switches is a NPN transistor having a base, an emitter and a collector, the base is connected to a corresponding output terminal of the control unit, the emitter is connected to a ground, and the collector is connected to a corresponding second switch through a third resistor; and

each of the second switches is a PNP transistor having a base, an emitter and a collector, the base is connected the third resistor, the emitter is connected to a corresponding constant current circuit and the collector is connected to a corresponding lighting unit of the LED light string.

3. The constant-current controller as claimed in claim 2 , wherein

each of the constant current circuits comprises a first resistor, a first capacitor, a second resistor, a first diode, a second capacitor, a second diode and a current limiting resistor;

the first resistor is connected to the AC voltage;

the first capacitor and the second resistor are connected in parallel and further connected in series to the first resistor, the second diode and the current limiting resistor;

the first diode is connected between an anode of the second diode and the ground; and

the second capacitor is connected between the anode of the second diode and the ground.