First, tiny project

[smudgepost]

Hi all, I was hunting around the web looking for some circuit designs and stumbled across this forum.

I wish to make a very very simple first circuit for a prop. It is simply a Red LED that pulses on and off, rather than a strict on/off blink, probably on 1/3 of the time, off 2/3 of the time but is smooth between on/off.

It needs a switch and to be as low power as possible, as I was hoping to hide a watch/button battery in it. Battery life 24-48 hours. The battery time is a minimum but my main restriction is circuit space, as it needs to fit in a watch case (roughly H 30mm x W 35mm x D 8mm)

I need to keep it small and was thinking I could achieve the pulse using capacitors but like I said this is my first project. Can you please help a new chap start a new hobby?

 

[MikeMl]

When you say "smooth between on/off", do you mean that the apparent brightness should ramp up and ramp down smoothly. If so, you are looking at a Pulse Width modulator driven by a slower ramp. Two opamps, a transistor, half dozen resistors, two big capacitors in addition to the battery. It will fit in a watch case only if you build it using surface mount parts. Are you up to building a custom PCB using Surface mounts?

 

[EN0]

I would use the TLC555CP, it is a low power timer that takes little current to operate. You can use the RC time constant formula to figure out how long you want the LED to remain on and how long it will remain off. How much current do you want the LED to run on? A typical 20mA would be out of the question, a watch battery would fizzle out soon enough.

 

[smudgepost]

Thanks for the input guys. I am not bad at assembling circuits from diagrams just no good at designing them, so yes I might consider a direct mount pcb. The TLC555CP sounds interesting and they're pretty easy to get hold of if I haven't got one.

Ramp up and down sounds about right, as blink on and off looks a bit harsh, possibly over two time periods, 2 and 4 seconds selected by switch. Many of the links I've found online are too broad with complex flashing patterns or the diagram links no longer work so currently head scratching..

 

[EN0]

Smudgepost,

What LED current do you want?

 

[smudgepost]

No preference, though I'll likely get components from Maplin Electronics so am limited to their led range and batteries.

Under review:

LED
Button batteries
Flashing 3mm LED

 

[smudgepost]

After your input, I have been looking more at the 555 timer. I hadn't realised earlier that it needs 5V to actually work, and this is my bottle neck. There are some nice simple breadboard circuits here, probably featured on youtube also.

My initial aim was to use a watch battery and I think they are either 1.5V or 3V.

**broken link removed** of the NE555N suggest 4.5V min, and 5V trigger voltage. As I am confined to space, I should set button batteries in series for 6V.

So I need to modify the circuit in the above link to work for 6V - darn am I rusty, he cries humbly to the ocean of wizards..

 

[tblo163]

Cheapest way is to buy a "rainbow" LED it pulses on and off,and changes colour.

 

[jakeselectronics]

If you have limited space and are going to use surface mount components, for the led/leds you can use PLCC-2 type.

I used them on my keypad interface PCB I made.
Take a look on my site
Jake's Electronics | Kits, Tutorials, Projects and More.

or search them on eBay. You can get some low mcd(brightness) ones pretty cheap.

There are stacks of ways you can do it and components you can use. This is just one suggestion.

I know some traditional people are against always reverting to a microcontroller, but it would be perfect for this situation.
- you can get your timing perfect for the on/off ratio.
- it can perform you fade in fade out function with a pwm
- some run off 3v
- 8 pin SMD chips are tiny
- they can sink/source up to 25mA which is enough to drive an led.
- minimal parts required, led, microntroller, 1 or 2 caps,battery, and a switch for function select.

Of course the downsides are you need the programming knowledge and hardware

 

[MrDEB]

the PIC is best choice but

the 7555 is the cmos version of the 555
will run on 2 - 18v dc
for tiny circuit you will need to go surface mount.
then use surface mount leds.

 

[tblo163]

Try this one

 

[MrDEB]

I got impression = slow fade

in one of the posts I think the op wanted a fade instead of on/off blinking?
here is a possable solution
not tested but should work.
using SMD (custom pc board) it can be built smaller than
L 30mm x W 35mm H 8mm
looked at malpin they carry the 7555 but trying to locate a SMD ???

 

[audioguru]

here is a possable solution
not tested but should work.

Please do not post circuits that blow up the LED because there is no current limiting.

EDIT: Also it won't work with a 2V supply because the transistor has a voltage loss of about 0.8V or more and an LED needs at least 1.6V.

 

[MrDEB]

oups forgot resistor

I used an example schematic from last year. instead of a single transistor it has a darlington etc.
suggest upping the supply voltage.

 

[smudgepost]

I'm still waiting on the transistor before building it but see the issue. I am basically going off your schematic MrDeb and plan to tweak where necessary. I was thinking of using either 3V or 6V. Should get transistors in a day. Couldn't find a TIP 120..

 

[MrDEB]

you don't need the tip 120

I copied n pasted from a larger schematic.
I thought I took that out??
for one LED all you ned is a 2n2222
sorry for the booboo
you still need to add a resistor to led depending on voltage.
see audiogurus post

 

[MrDEB]

look at my LT spice post

reconfiguring so LED turns on slower

 

[audioguru]

I am basically going off your schematic MrDeb.

Don't make MrDEB's defective circuit:
The timing resistors are backwards so it will ramp on the LED fairly slowly then will try to switch off for such a small amount of time that the LED will appear to be on continuously.
But since there is no current-limiting the LED will burn out.

 

[smudgepost]

This thread has grown and I'm really pleased. I want to try building them all! I am stuck on time for version 1.0 of this so will likely stick with 555 but if the newer 7555 idea works better then I might just use that. The keypad interface is very interesting and will read more tomorrow. I will likely run with the double 555 circuit initially and use a 2n2222 as suggested mainly as these are the parts I've got. If you could help me understand what I need to do to fix it before I burn it that would be great.

As a recap, I'm limited on space so opted to use button batteries so could probably squeeze upto 6V inside. That' my only bottle neck in the planning. I also bought a 5mm and 3m super bright white LED for the flash and am plan to use a piezo for the sound. Oh and yes I would prefer a steady blink to a bold on/off flash.. if possible.

I know some traditional people are against always reverting to a microcontroller, but it would be perfect for this situation.
- you can get your timing perfect for the on/off ratio.
- it can perform you fade in fade out function with a pwm
- some run off 3v
- 8 pin SMD chips are tiny
- they can sink/source up to 25mA which is enough to drive an led.
- minimal parts required, led, microntroller, 1 or 2 caps,battery, and a switch for function select.

Of course the downsides are you need the programming knowledge and hardware

Hardware - easy, know-how - not so easy..

 

[audioguru]

Button batteries have such a low capacity that the LED will turn on for only a few times. You must blink an LED fairly dimmly and for very short durations for some button cells to last for a few hours.
Button batteries power items with an LCD display that does not produce light so its current is very low.