NEED HELP! Simple DC circuit

This is a latching circuit.

Start thinking about your switches not as driving components directly but only as means of providing a 5V (high) or 0V (low) level. So you want a High state when switch 1 is closed until switch 2 or 3 is closed. In other words: 1 sets the output to high and (2 or 3) resets it. Take a look at the following schematic which includes an interactive demo :

The website explains how the individual gates can be built from transistors. You might need to invert the input/output and OR the 2 &

3 inputs together. Again, instructions can be found on the website. If your transistor count gets too high you might want to look at the 74HCxxx or similar ICs that contain gates; I don't know how much current they can supply, so you might still need to provide a transistor to drive your light.

you need an R-S flip flop. (R = Reset), (S= Set). they create a latching effect to the Q outputs. just about any FF with R&S with work.

Or, you can use 2 Transistors of NPN types. Look here, this is what you can do with transistors which is a Monostable set/reset flip flop.

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Here's a circuit that will do what you described. I assumed a relatively low current requirement for the buzzer.

PNP 2N3906 +5 -+--- ----------[1K]--------+ | e\\ /c | | --- [LED] | | | | | [SCR1] P0118MA2AL3 | | Pb3 /| | | ___ | | `----------+---o o---[47K]---+ +-----------+ | | | | | Pb2 | | | ___ | | +---o o---[Buzzer]-----+ | | | | | | | | +--->|-----+ | | | | D2 | | | | | Pb1 | | | | | ___ | /c | | +---o o---[1K]---|---| NPN | | | | \\e | | +--->|-----+ |2N2222 | | D3 | | | | D1 | | | `--->|---+---[.1uF]---+ | | | | | | | | | | [1K] [10K] | | | | | | Gnd ----------------+------------+-----+--------+

Pressing Pb3 turns on the SCR, which stays on, lighting the LED, until current through it is interrupted. The PNP is biased on through D1 and the 1K resistor. When either PB1 or PB2 is pressed, a pulse is generated through the .1 uf capacitor, which briefly biases D1 and the PNP off, interrupting the current through the SCR, so it turns off. An SCR (Silicon Controlled Rectifier) is a device that once turned on by an external signal at its gate, stays on even though the external signal goes away, until current through it is interrupted.

Pb1 turns the NPN on while it is pressed, connecting one side of the buzzer to (-). If Pb2 is also pressed, the other side of the buzzer is connected to (+), and it sounds.

The SCR is mouser part # 511-P0118MA2AL3, and the diodes are 1N4148 or 1N914 If the buzzer draws more than about 200 mA, substitue a relay for the buzzer in the circuit, and use the relay contacts to complete the circuit to the buzzer. If this is a "hardware store" variety buzzer, you'll need the relay with a 1N4148 diode across it backwards, and will need to add a 1N4004 diode across the buzzer too (also backwards). "Backwards" means the cathode end (the end with the stripe) connects to (+) instead of (-).

Ed

You might need a smaller resistor in series with the LED. I think the SCR holding current is around 5mA and you only have about 1 mA in the LED. The SCR will drop 1.5 or more, and the PNP will drop another .5 and the LED drops 1.8, so you only have 5-(3.8) = 1.2 volts across the resistor, or 1.2 mA.

-Bill

You're right - thanks for picking up on it. That 1K should be 81 ohms. I made 2 mistakes - I figured 220 ohms, then wrote 1K. And I didn't figure in the drop in the SCR. The SCR would hold at 220 ohms, but current would be only a little over 5 mA, so that 1K needs to change to

81 ohms. That'll give ~ 15 mA with a 5v supply.

Ed