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Power Sequencer


A common problem in audio systems is the speaker "thump" that occurs if all of the equipment is switched on at the same time.  One way to eliminate this thump is by turning on the audio sources that feed the amplifiers (such as the sound board, etc.) before turning on the amplifiers.  The power sequencer described here has two outputs that are turned on and off in a sequence that prevents speaker thump.  

 When the switch is turned on, sound board outlet is turned on first, and then the amplifier outlet is turned on a few seconds later.  When the switch is turned off, the amplifier outlet is switched off first, and then the sound board outlet is turned off a few seconds later.  See the timing diagram below.

 Technical Details

A schematic of the power sequencer is shown below.  The timing sequence is generated with a PIC10F202 microcontroller running a very simple program.  The PIC outputs control two solid state relays that I found at All Electronics for about $10 each.  They are each capable of switching up to 18A, which is more than enough that I need.

  Parts List

Part        Description                        All Electronics Part #
U1            7805 Voltage Reg.               7805T    
U2            PIC10F202                             www.microchip.com
C1             .1uF Capacitor                       104D50
R1            22k ohm resistor                   11K-1/4
Q1,Q2      Solid State 18A Relay          SRLY-18
J1,J2       120VAC Outlet                       ACS-3
T1             Wall Transformer                  DCTX-937

120VAC is fed into the sequencer.  The input wiring must be capable of handling enough current to feed both sequencer outputs.  The 120VAC is converted to 15VDC with a "wall wart" transformer.  That 15VDC is then regulated down to 5VDC using a 7805 linear voltage regulator.  The PIC software is very simple.  All it does is look for the switch input (pin 8) to go high.  When it does, it turns on the two solid state relays in the appropriate sequence.  When the switch goes back low, it turns the solid state relays off in the opposite order (refer back to the timing diagram above).