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QUESTIONS & ANSWERS INTERCOM PARTS LIST PART SW1 FUSE T1 D1, D2 C1, C2 U1 C5, C7 SPEAKERS C3 SW2 RLY1, RLY2 R1 IC1 IC2 C4, C8 C6 R2, R3 R4 R5 R6 R7

DESCRIPTION JAMECO # SPST POWER SWITCH 317287 5x20 MM CLIP 1712608 115V/12VCT T WO AMPS 29226 1N4004 DIODE, 400V, 1A 35991 3,300 µF, 16V 198898 VOLTAGE REG 5V, 1.5A 924570 0.1 µF, 50V, +80-20% Z5U 15272 4 OHM, 4” DIA 2099614 220 µF, 16V 30496 PUSHBUTTON, MOMENTARY 315660 RELAY, DPDT, 5V COIL 2135291 50K POT, PANEL MOUNT 255434 LM386N AUDIO AMP 24125 LM358AN DUAL OP-AMP 120862 10 µF, 16V, RADIAL 198839 1 µF, 50V, RADIAL 29831 1K, 5%, 1/8W 10K, 5%, 1/8W 4.7K, 5%, 1/8W 10 OHMS, 5%, 1/8W 100K, 5%, 1/8W ■ FIGURE 5.

DC-TO-DC REGULATOR

QI would like to build a well regulated step-down DC-to-DC power regulator built with minimal parts (from Mouser), with an input of 12 VDC 200 mA and an output of 3 VDC 10 mA.

Can you please help create such a schematic?

I have several 12 VDC wall adapters rated at 200 mA

■ FIGURE 4.

(purchased from goldmine-elec.com; #G14801) which would power-up several kits, each being rated at 3 VDC @ 10 mA — i.e., one is this five LED random/sequence flasher whose prerequisite for operating current is 10-16 mA because of the CMOS VLSI single chip-on-board contained in the circuit (any higher current would destroy the chip-on-board).

— Michael Williams

ASince efficiency is not a consideration, a shunt regulator is the simplest solution. Take a look at Figure 6. The LM431A is rated at 36 volts and 100 mA max, and the reference voltage is 2.5V ±2%. The theoretical value for R2 is 400 ohms, so the 20 ohm pot allows you to get the right value and also trim out the tolerance. The load current is limited to 15 mA by R1; if the load tries to draw more current, the voltage will reduce.