mark@mips.COM (Mark G. Johnson) (09/30/89)
In article <4377@wpi.wpi.edu> reynhout@wpi.wpi.edu (Andrew Reynhout) writes: > > I need a circuit that will, on application of power, cause a delay of 5-20 >seconds before allowing current to flow through (a different part of the circuit) > Basically, it should take, say, 12v, wait 10 seconds, then trip a relay. Here's a non-deceptively simple circuit that will do the trick. Time delay is about 7 seconds, and parts cost is $3.00 in single-unit quantities from mailorder houses like Digi-key. There are 3 functions: (1) sense whether power has been applied; (2) delay 5-15 seconds; (3) switch the load. The ckt below combines (1) and (2) in an input network, then drives an output stage for (3). (r1 and c1), (r2 and c2) provide the delay. Zener diode DZ1 makes the ckt wait until V+ has reached its final value. The Zener voltage of DZ1 should be about 2.5 volts less than the supply voltage V+: V+ zener rating 15V 12V (1N4742) 12V 9.1V (1N4739) 6V 3.6V (1N4729) 5V 2.4V (four 1N4148's in series) Once the supply voltage stabilizes, c2 eventually reaches 1.7 volts. At that time, NPN transistors Q1 and Q2 turn on, which supplies current to the load. Q1 should be a high-beta device to keep its base current requirements small; 2N5089 or 2N5210 are good choices. Q2 is a general purpose NPN having a current rating commensurate with the load. When the power shuts off, clamping diode d1 discharges c2 quickly, to de-energize the load. You might add another such diode across r1, to also discharge c1. ---+---------+---------------------------+------------+------ V+ | | | | | +---+ | | | |33K| r1 | | | +---+ | | d1 | | | | - | +-----------+ | | ----- + | + | 47 uF | | |1N | ----- ----- | +-------+ | 41| |DZ1| ----- c1 | | Load | | 48| ----- - | | | to be | ----- - | | | | switch| + | | GND C / | ed| | +---+ / +-------+ | |75K| r2 B |/ | | +---+ +------------| Q1 | | | | |\ | +---------+-----------+ \ C / | | E \ / | + | 47 uF | B |/ +---+ ----- +---------| Q2 |3.3| ----- c2 | |\ r3 |meg| - | +---+ \ +---+ | |20K| E \ | | r4 +---+ | | | | | GND --------+-----------+---------------+------------+-------------- GND -- -- Mark Johnson MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086 (408) 991-0208 mark@mips.com {or ...!decwrl!mips!mark}