# Latching circuit that delays the on state for different outputs

First, I'm very inexperienced in designing circuits like this, so I apologize if this seems trivial. I'm basically trying to create a startup sequence effect, where LEDs turn on in a delayed order. Here's the effect I'd like to create.

• Push momentary switch once
• Wait ~2 seconds, then turn on a few LEDs (latched on)
• Wait another ~2 seconds, then turn on a few more LEDs (latched on)
• 2-4 additional on events to turn on LEDS in sequence
• Push momentary switch again to turn off all of the LEDs

I'll be running either 9v or 12v. I'd prefer to have n-delayed switches (well, at least 5 or 6) chained together with delays between each event (0.5 sec and 5 sec) to create the power up sequence effect I'm looking for. The delay between two events is constant, but it'd be optimal to have some control over the delays individually. For instance, the delay between 1 and 2 may be 2 secs, but the delay between 2 and 3 may be 1.2 secs. The specific timing will take a bit of experimentation. Each of the on events will have about 4-6 LEDs, so not much power is needed on any individual output.

Seems like a few 555 timers should manage it, but I've gotten a bit lost trying to solve this. Any help is greatly appreciated.

• This would be trivial to do with a small 8-bit microcontroller like a PIC16. Is there any reason you can't take this route? – tcrosley Apr 28 '14 at 2:13
• An excellent idea... Although I've never used a PIC16. Any recommendations on learning more about using them? Or, can you point me in the right direction on using it for something like this? – josh Apr 28 '14 at 3:50
• I'm not exactly sure how to give the momentary push switch the conditional ability to start or reset the system, but if you google around you may find what you're looking for in building a custom binary counter. You just have to write out the truth table, solve it and implement it in flip flops. Note that this is probably going to have a fairly big footprint; you should probably go with other recommendations involving microcontroller designs. +1 for pic16 and arduino. – Sean Boddy Apr 28 '14 at 5:06
• @josh I expanded my PIC16 suggestion into an answer. – tcrosley Apr 28 '14 at 6:11

Your best bet is probably to get an arduino. Depending on your total LEDs (current draw) you can either hook them up directly with resistors or use an external power supply with transistors. Good luck.

• I considered something like an Arduino or Raspberry Pi, but that's a lot of overhead for something this trivial. And, I want to keep the footprint minimal, because space is a premium in this instance. – josh Apr 28 '14 at 3:52
• @josh An arduino nano compatible board might be an option, at 1.85cm x 4.3cm, and $5-$10 on ebay? They don't require a separate programmer, which can be a big cost saving if you are only making one device. – Chris Johnson Apr 28 '14 at 7:41

To keep parts count to a minimum, I suggest using a small 8-bit microcontroller like the PIC16, in one of the newer variants, particular the PIC16F1788. Some of its specs are:

28 KB flash memory, 2 KB RAM (much larger than earlier PIC16s)
runs on 2.3v to 5v
comes in a 28 pin DIP for prototyping, and surface mount for PCB's
can source or sink up to 25mA on each output pin (max, keep well below that)
internal oscillator, so you don't need to wire in a crustal
costs \$2.66 in single quantities


Because of the internal oscillator, and the ability to drive the LEDs directly (no drive transistors), all you will need is a power supply, bypass cap, micro, and LEDs plus resistors for each.

I'd run the micro at 5v, and use that same rail for my LEDs. If you run your LEDs at (for example) 15 mA, you can connect the anode end to 5v through a resistor, and the cathode end directly to one of the PIC I/O pins; you will then configure the pin as "open-drain" and set it to 0 to turn on the LED.

You will also see a lot of schematics where the LEDs are wired just the opposite; the cathode end connected to ground, and the anode end is connected through a resistor to an I/O pin. In this case, the I/O pin is configured as push-pull, and sources current (I/O pin set to 1 to turn on the LED). Either configuration will work just as well.

PIC16F1788 features

PIC16F1788 datasheet

PIC16F1788 Digi-Key page