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I would like to flash an LED when my 5V power supply is turned on to indicate my Raspberry Pi is booting. Then, when booted and my application is running, I'd like to make a GPIO high (or low) and have the same LED constantly illuminated. This would indicate the system is ready for us.

The Raspberry Pi will be in a case and the would be no other way for a user to determine it is ready to use.

So far, I've made a simple 555 circuit to flash the LED but can not work out where I can add a transistor to be controlled by GPIO to either bypass the 555 or change the R1 or C1 values so the flash rate is 1kHz or more and so imperceivable.

Is there a simple addition I can make to the circuit below to achieve this?

555 timer schematic

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  • \$\begingroup\$ Having a LED that blink while booting is giving mostly false reassurance to a user. I suggest a full-on LED while booting, turning off at end of boot, blinking when the thing is active. Much more truthful. \$\endgroup\$
    – fgrieu
    Commented Mar 22 at 18:38

3 Answers 3

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The NE555 has a reset pin. You can control it with GPIO.

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  • \$\begingroup\$ I think I can see how I could use the reset pin to turn the LED off via GPIO, but is it possible to turn the LED on using it too? \$\endgroup\$
    – edlea
    Commented Mar 22 at 17:38
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    \$\begingroup\$ @edlea Connect the LED so that it is permanently lit when NE555 is in reset. \$\endgroup\$
    – Justme
    Commented Mar 22 at 17:49
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    \$\begingroup\$ I.e. connect the anode to your supply voltage through an appropriate resistor and the cathode to OUTPUT so it sinks the LED current (LED lit when OUTPUT is low) rather than sources it. \$\endgroup\$
    – vir
    Commented Mar 22 at 18:12
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If you already have the circuit built and can't change it, this might work:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Driving a LED with no resistor is never a good option. \$\endgroup\$
    – Justme
    Commented Mar 22 at 18:03
  • \$\begingroup\$ @Justme Yeah. You're right. \$\endgroup\$
    – MOSFET
    Commented Mar 22 at 18:06
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You could also use a flashing LED as a simple flash circuit and bypass it with the GPIO after startup.

schematic

simulate this circuit – Schematic created using CircuitLab

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