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I have been trying to get this circuit working for sometime. Mechanically it works but when the potentiometer through which the capacitor charges is set to maximum the maximum timer period before the LED lights, or Buzzer goes off is roughly 12 seconds. As I don't have a smart phone to capture the schematic straight from the magazine I will show its reconstruction here. It is a fairly simple circuit so I can't understand what I must be doing wrong.

schematic

simulate this circuit – Schematic created using CircuitLab

The only difference in the schematic above and that from the magazine is that R1 is actually a POT which controls the timer interval before D3 goes off. I just substituted the maximum value of the POT (when rotated fully clockwise) so as to simulate it with its maximum timer interval.

The explanation the magazine gives for the circuit is such that when connected C1 charges up via current taken from R1, R2, D1, D2 and the base emitter junction of Q3. Because Q3 is providing current to the charging capacitor no current flows through Q2. When the capacitor is fully charged no further current can be drawn from the components connected to it and so Q3 swictches off turning on Q2 and hence the LED or buzzer. closing SW1 restarts this process. I have connected this circuit up on a spring board and it only runs for 12 seconds, not 3.5 minutes (with maximum POT value or R = 1MOhm).

If I had a smartphone I would photograph the spring board but I have constructed more complicated projects on the board than this and so can't see how my connections could be wrong. I have even gone through with a multimeter testing the components and noted that the capacitor did charge up causing Q3 to switch off and Q2 to switch on only this was much quicker than 3.5 minutes. Any tips or hints?

Thanks,

Simon.

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  • 1
    \$\begingroup\$ Try increasing the value of R4. A lot. \$\endgroup\$ – brhans Nov 18 '16 at 3:34
  • \$\begingroup\$ I tried moving it up to 100k which made no real difference. It must be some printing error with the magazine. No way could this produce a 3.5 minute delay. I might try the circuit combination provided below. \$\endgroup\$ – burton01 Nov 18 '16 at 22:27
  • \$\begingroup\$ I was thinking in the order of 470k or more. \$\endgroup\$ – brhans Nov 19 '16 at 2:12
  • \$\begingroup\$ Yeah I might give that a crack. I just think its absurd that it doesn't work as is considering all other projects worked pretty much the first time after connecting. I can't understand why this is only running for 12 seconds when it says 3.5 minute timer. Its making me think I might have to do some circuit analysis to prove to myself that it should/shouldn't work as specified. \$\endgroup\$ – burton01 Nov 20 '16 at 1:39
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This seems to work pretty well, it's got a fairly crispy edge, and I put the switch in series with the positive rail so the battery won't be wasted when the LED is off.

The LTspice circuit list follows the drawing just in case you want to play with the circuit.

enter image description here

Version 4
SHEET 1 880 680
WIRE -736 -2176 -880 -2176
WIRE -448 -2176 -736 -2176
WIRE -272 -2176 -448 -2176
WIRE -80 -2176 -272 -2176
WIRE -736 -2144 -736 -2176
WIRE -448 -2128 -448 -2176
WIRE -272 -2128 -272 -2176
WIRE -80 -2128 -80 -2176
WIRE -80 -2032 -80 -2048
WIRE -80 -1936 -80 -1968
WIRE -272 -1888 -272 -2048
WIRE -144 -1888 -272 -1888
WIRE -272 -1856 -272 -1888
WIRE -448 -1808 -448 -2048
WIRE -336 -1808 -448 -1808
WIRE -448 -1776 -448 -1808
WIRE -736 -1728 -736 -2064
WIRE -688 -1728 -736 -1728
WIRE -560 -1728 -608 -1728
WIRE -512 -1728 -560 -1728
WIRE -880 -1680 -880 -2176
WIRE -736 -1680 -736 -1728
WIRE -560 -1680 -560 -1728
WIRE -880 -1552 -880 -1600
WIRE -736 -1552 -736 -1616
WIRE -736 -1552 -880 -1552
WIRE -560 -1552 -560 -1600
WIRE -560 -1552 -736 -1552
WIRE -448 -1552 -448 -1680
WIRE -448 -1552 -560 -1552
WIRE -272 -1552 -272 -1760
WIRE -272 -1552 -448 -1552
WIRE -80 -1552 -80 -1840
WIRE -80 -1552 -272 -1552
WIRE -880 -1488 -880 -1552
FLAG -880 -1488 0
SYMBOL LED -96 -2032 R0
WINDOW 0 -45 32 Left 2
WINDOW 3 -96 64 Left 2
SYMATTR InstName D1
SYMATTR Value NSCW100
SYMBOL Misc\\battery -880 -1696 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 9V
SYMBOL res -96 -2144 R0
WINDOW 0 34 48 Left 2
SYMATTR InstName R6
SYMATTR Value 270
SYMBOL cap -752 -1680 R0
SYMATTR InstName C1
SYMATTR Value 200µ
SYMBOL res -752 -2160 R0
WINDOW 0 47 55 Left 2
WINDOW 3 38 80 Left 2
SYMATTR InstName R1
SYMATTR Value 1meg
SYMBOL npn -144 -1936 R0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL res -256 -2032 R180
WINDOW 0 -43 61 Left 2
WINDOW 3 -48 34 Left 2
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL npn -336 -1856 R0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res -432 -2032 R180
WINDOW 0 -38 65 Left 2
WINDOW 3 -45 37 Left 2
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL npn -512 -1776 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL res -576 -1696 R0
SYMATTR InstName R3
SYMATTR Value 820k
SYMBOL res -592 -1744 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 1meg
TEXT -852 -1520 Left 2 !.tran 250 startup uic
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