# Why isn't this single pushbutton on/off circuit working?

I built this about four times, replacing the transistors every time, carefully measuring all resistors and capacitors and putting a .1uf cap from the output to the positive. The LED light goes from the output to a 470 resistor, and to positive like the circuit called for. When I power it on, the LED just stays on, and if I press the button, nothing happens whatsoever. The LED is just ON.

Just two notes so as to not get confused with the picture:

• the 2 10k resistor wires aren't touching and the 470 ohm that may look like its grounded is to the positive.
• Also, the middle is the BC557, and the two outers are the 547'

Edit:

• When the button is pressed, the voltage across the LED drops for 2.5Ms from 2.4 to 2.0 volts
• Are you certain that the switch is installed properly? It looks like you have one with four pins, and if you accidentally turn it 90 degrees you end up with a short between the terminals. Sep 25 '13 at 0:11
• I checked and it was properly installed Sep 25 '13 at 0:42
• Does it behave if you remove everything connected to the output (your .1uf cap and led) and just measure the output voltage? Sep 25 '13 at 7:40
• TO-92 packages used to be tricky : there used to be 2 or 3 pinouts of the same transistor e.g. BC214 and BC214L. You'd like to think they eliminated that insanity before the BC547/557 but it might be worth a check... Sep 25 '13 at 8:03
• Does your switch itself behave as expected? It could be a buttons that doesn't hold conducting when pressed. Sep 25 '13 at 9:31

Sometimes, after you've carefully checked everything, it turns out to be someone else's fault. In this case I think the original design is bad. I've tried it in Circuitlab (should be here) and I've tried it in Falstad:

1 5.0E-6 85.40587625261516 60 5.0 50
r 128 112 128 208 0 470000.0
r 128 208 128 288 0 470000.0
r 400 96 400 192 0 1000000.0
r 400 192 400 272 0 100000.0
r 272 208 208 208 0 10000.0
r 272 224 272 288 0 10000.0
r 272 288 272 368 0 470000.0
r 272 288 336 288 0 1000.0
t 336 288 400 288 0 1 0.5854708958314027 0.587099911972777 100.0
t 208 208 176 208 0 1 0.587957264003006 0.588497761400722 100.0
w 272 224 272 208 0
w 400 304 400 368 0
c 128 288 128 336 0 1.0E-6 0.02827590396558895
w 176 192 128 208 0
w 176 224 176 368 0
w 128 336 128 368 0
t 304 192 272 192 0 -1 -0.5251112318758011 -0.5895458111379241 100.0
w 400 192 304 192 0
w 272 176 272 112 0
w 128 96 128 112 0
w 272 96 272 112 0
w 400 96 272 96 0
w 272 96 128 96 0
w 128 368 176 368 0
w 176 368 272 368 0
w 272 368 400 368 0
s 208 288 240 288 0 1 true
w 272 288 240 288 0
w 128 288 208 288 0
v 528 160 528 96 0 0 40.0 9.0 0.0 0.0 0.5
w 528 96 400 96 0
w 528 160 528 368 0
w 528 368 400 368 0
w 400 272 464 272 0
o 33 64 0 35 0.009765625 9.765625E-5 0 -1
o 28 64 0 35 0.3125 9.765625E-5 1 -1


In both cases it doesn't respond to button presses (this is rather hard to simulate in circuitlab)

Without any more "theory of operation" info (no I'm not going to bother watching the video), I would say there is a design problem. A more conventional "bistable multivibrator" might work better.

• +1 for problems with the circuit design. I have a vague recollection that a problem with this circuit, or one very similar, has come up before on electronics SE. I recall that an answer to the previous question pointed the finger at non-ideal transistor behaviour (perhaps Miller capacitance?) triggering the circuit immediately when power is turned on. Apr 23 '14 at 15:28
• And I've just found the answer I was thinking of: electronics.stackexchange.com/a/45195/1276 Apr 23 '14 at 15:39

Troubleshooting 101: if the LED is on, according to the schematic and assuming the circuit matches the schematic, all three transistors are "on" which implies that the voltage on C1 is about 0V.

When the switch is momentarily closed, the base of Q3 is connected to the capacitor and thus, 0V. Thus, Q3 should turn off, extinguishing the LED and turning off the other transistors.

So, my recommendation is that you start troubleshooting by putting a 'scope on the base of Q3 and see what is happening when you press the switch.

• Nothing, the base of Q3 is always HIGH Sep 25 '13 at 0:42
• The LED dims for a fraction of a second when pressed Sep 25 '13 at 0:45
• Ok.Across the LED, I get a small dip,the voltage lowers abruptly, stays for 2.5Ms, and slants up for 2.5Ms. THis is happened right when the button is pressed. The voltage looks like it drops from 2.5MAX to 2.0Min in this period Sep 25 '13 at 0:51
• @skyler, what does the voltage on the positive plate of C1 look like during this process? Sep 25 '13 at 1:43
• @skyler, generally speaking, the voltage across a capacitor does not "spike" since the capacitor current is proportional the rate of change of voltage. So, if the capacitor voltage "spikes", the current must be relatively larg... er... enormous. What should happen is that, when the switch is pressed, the voltage across C1 should initially be about zero, rise relatively slowly and up to about the supply voltage and stay there. Either you have a wiring error or a defective component (maybe more than one). How would you go about determining which is the case? Sep 25 '13 at 1:56

I've simulated that circuit in LTspice XVII and it did not work. Then I found another design (that uses 5 V input) and it did work as intended. As such, I would say it truly is a design flaw.

The following figure shows the simulation in LTspice XVII. I used a voltage controlled switch. V(vsw) makes the switch close while it is 1 V. V(out) shows the output; notice that it toggles each time the switch closes.