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I have this schematic:

555 dual led flasher

which supposedly makes the two LED flash alternatively with a period of 1.5s The thing is that the one connected through the NPN transistor flashes, but in it's "off" state, still has about 6V going through it, and the other one doesn't flash and it's always powered with 12V.

I double checked my connections, everything is fine. I don't need resistors for the two LEDs because they are at 12V

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    \$\begingroup\$ You cannot run the 555 from 6V and the LEDs from 12V. Run both from the same supply and you should be OK. Note however that your LEDs MUST have series resistors ...you show none in the schematic. It may be you are using LEDs that already have series resistors in the assembly, but you should check. \$\endgroup\$
    – Jack Creasey
    Jun 13, 2018 at 16:09
  • \$\begingroup\$ What do you mean by a "12 volt LED"? LEDs have to be current limited or they blow up. Do these have resistors in series built in? \$\endgroup\$
    – zeta-band
    Jun 13, 2018 at 16:11
  • \$\begingroup\$ Try this circuit instead, and power everything from 12V: circuitdiagramworld.com/uploads/allimg/201411/… \$\endgroup\$
    – DerStrom8
    Jun 13, 2018 at 16:14
  • \$\begingroup\$ PNP needs a level shifter so use a 12V clock \$\endgroup\$
    – Tony Stewart EE75
    Jun 13, 2018 at 16:16
  • \$\begingroup\$ These LED probably have some current limiting built it, because I can power them from 12V just fine without a current limiting resistor. Thing is the important part is the 12V power supply, I have no problem powering all from 12V, but will the 555 take it? \$\endgroup\$
    – sundaysfantasy
    Jun 13, 2018 at 16:24

2 Answers 2

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I assume you mean that the LEDs have internal current limiting so that they can be operated from 12 volts.

When the 555 output is High, its output will be about 6 volts - well below the 12 volts on the BC327 emitter, so the BC327 will always conduct, allowing its LED to light.

When the 555 output is Low, the current through the BC327 emitter-base junction, and its 220 Ohm resistor, is apparently enough to pull the 555 output up enough to allow some base current into the 2N2222, so it will draw some current through its LED.

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This should work for you.

  1. The problem with the PNP drive is that the 555 output would need to get to about 11.5V to turn the PNP off, and it doesn't get there. Bipolar 555s don't get that close to their supply voltage even if you increased the 555 supply to +12 so that is not a quick fix, and it could expose your 555 to potentially fatal transients if the 12V is an automotive supply.

  2. You are measuring voltage across the 'OFF', but that doesn't matter if it appears off, there is no significant current flowing, so don't worry about it.

In the below circuit, I've retained the NPN drive and added a second one, with a third transistor to invert the drive signal. When Q2 is 'on' it diverts the ~27mA base current away from Q3 so Q3 goes off (at the same time that Q1 turns on). I've shown the internal resistors in the LEDs so as not to confuse future readers.

schematic

simulate this circuit – Schematic created using CircuitLab

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