NE555 Based Variable ON/OFF Timer (Updated 2018)

Welcome,

some of my friends including me have made D.I.Y spot lights for our bicycles but as usual they got jealous watching other branded lights.
Why? Because those lights have a strobe function! lol
Each of my friends have made he's own light with different configuration housing, bulbs, batteries, operation voltage and amperage. So, I needed to build one circuit to fit in every light with no extra effort. Here is the answer, the 555 IC it's the perfect cheap choice and it will do the job for all the lights. Off course we can buy ready made ones and also cheaper but making your own from the scratch it's fun.
Also i would like to point out that the uses of these little things are endless. it can be a bicycle strobe light, christmas lights, car strobe light and so on. Just use your imagination!

A few words about the mighty 555 IC.

It can operate from 3VDC to 16VDC MAX. It can deliver 200mA output from pin 3 so it's ok to drive a few typical led.
Even so, 200mA it's the maximum output therefore the IC safer at MAX output, no good!A better solution is to use a transistor to deal with the LOAD driven from the 555 IC and let the second do it's job and by that I mean, the counting for the strobe operation.I'm not gonna go any deeper concerning the operation of the 555. There is plenty info out there if someone who is interested to learn everything about the 555's operation. My intention is to help the beginner to make his own 555 strobe with basic info with less confusion, I hope!I will be happy if I can help with this instructable.
So, lets get started...

Step 1: OUTPUT LOAD & TOOLS

Add a boost to your 555
OUTPUT LOAD & TRANSISTORS - which one is the best for the job?
Here are some transistors from Low-power to Hi-power where can be used in this case.
LOAD = is the Amperage (A) the bulb,led draw when is on. 1A = 1000mA.

For 200mA LOAD => BC547 NPN
For 500mA LOAD => BC337, 2N1711 NPN
For 1,5A LOAD => BD135 NPN
For 3A LOAD => TIP31,BD241 NPN
For 4A LOAD => BD679 NPN
For 5-15A LOAD => TIP3055 N-gate (it is NOT recommended for this article's PCB because the traces are too thin and too close to each other to handle 5A > load).
Tip: never use a 500mA transistor for 500mA load without heat sink. It's best to use 1A transistor instead.

TOOLS NEEDED
Soldering iron. Not more than 25W
Solder Wire 0.5mm – 1.0mm will do
Soldering sponge
Jel-flux for the soldering
Small wire cutter
Drills = 0,7mm mainly & 1mm for the wires and the Q1 transistor
Mini hobby Handheld drill machine
Digital multimeter

Step 2: 555 As 1:1 On/off Cycles

PCB – Printed Circuit Board for 1:1 on/off time
The pcb is small enough to fit almost in any D.I.Y light enclosure.
You can download and print the PCB layout with the help of any graphics software that it can re-size the image at the print preview like corel photo-paint. The dimensions must be 21,5mm x 32mm at 72dpi resolution.
Print the PCB as is, remove the copper using any chemical technique you wish, use as thin drill as you can to open the holes,apply same jet-flux on copper is it will help when soldering and then turn it up side down to place the components. Pay attention when placing the components with polarity like the D1 diode and C1 capacitor. For the led, the long terminal indicates the anode (positive +). For the Q1 transistor see the schematic and off course check the 555. There is a round dot on top of the 555 near pin 1 indicating the pin number (1).

PARTS LIST – for 555 1:1 on/off time
All resistors 1/4 W
R1 = 1K
R2 = 10K
R3 = 1K
R4 = 680 for 5mm red led. 470 for white 5mm led
D1 = 1N5817 Schottky diode
D2 = LED RED 5mm or WHITE LED 5mm
C1 = 33uF / 25V electrolytic capacitor
C2 = 10nF
Q1 = BD135 NPN transistor
IC1 = 555 (NE555), 8 pin din (case)
PCB = about 25mm x 35mm
some thin wire
COST = not more than 4 euro

OPERATION & ABJUSTMENT – for 555 1:1 on/off time
Because of the presence of D1 Schottky diode as reverse polarity protection you will notice a difference between input and output of about 0,3 - 0,5V. That's normal for Schottky diodes.
It's better to protect the circuit from reverse polarity than burning everything.
To adjust the output in hertz=cycles per second (strobes) it only requires to replace the C1 capacitor. For shorter strobes use smaller capacitor in uF while for longer strobes use larger capacitor in uF.
If C1=47uF then it's about 1 Hertz (1 strobe per second). If C1=33uF then it's about 2 Hertz and so on. That's all!

Step 3: 555 With Variable On/off Period of Time

Here is a schematic for variable on / off time using 2 trimmer.

### UPDATE: from 9/12/2012 all files of this section have been updated due to previous incorrect files ### my apologies!

SCHEMATIC & PCB 2(A),2(B)
Download the 2(A) PCB & components placement image if you are about to use 10mm horizontal trimmers. PCB dimensions are h=31mm x w=37mm
Download the 2(B) PCB & components placement image if you are about to use 10mm vertical multi-turn trimmers,those are more accurate and also save some space from the PCB. The dimensions are h=32mm x w=33mm.

ABJUSTMENT – for 555 with variable on/off period of time
It's easy to build and very versatile, because if more time is needed all it requires is to replace the C1 capacitor with larger value in uF.
POT1 is used for the active period of time (on).
POT2 is used for non-active period of time (off).
Again, you can use any NPN transistor departing on the Amperage required.
The operation voltage is 5 – 15VDC.

PARTS LIST - 555 with variable on/off period of time
All resistors 1/4 W
R1 = 1K
R2 = 1K
R3 = 470
POT 1,2 = 100K trimmer OR multi-turn trimmer potentiometers
R4 = 680 for 5mm red led. 470 for white 5mm led
D2,3 = 1N4148
LED RED 5mm or WHITE LED 5mm
C1 = 10uF / 25V electrolytic capacitor
C2 = 10nF ceramic capacitor
Q1 = BD241 NPN transistor
IC1 = 555 (NE555), 8 pin din (case)
COST = not more than 6 euro

I hope this instructable was helpful and again if you have any suggestions, comments, ideas or questions please do so.

Step 4: Updated PCB Version 2018

Here is an updated PCB version of the LM555 based timer which can accommodate one turn potentiometer trimmers or multi-turn trimmers for best accuracy depending your needs.

Also, because C1 electrolytic capacitor is responsible for the time period, there might be a need to replace it more than ones with an other value. For the ease of use and for the sake of the PCB, C1 replaced with 2-Pin PCB Screw Terminal Block Connector. All we have to do now is to screw the C1 to the connector avoiding the peeling of it and straining of multiple times the PCB from high heat.

Remember the rule for C1:

C1 (electrolytic capacitor) is responsible for the maximum time the circuit can turn ON / OFF.

Low capacitance value say 1uF = sort time intervals.

High capacitance value say 100uF = longer time intervals.

Adjusting the timer:

POT1 (potentiometer): set the desired period of time when the circuit will turn ON a connected device (within the maximum limit of time C1 can give).

POT2 (potentiometer): set the desired period of time when the circuit will turn OFF a connected device (within the maximum limit of time C1 can give).

If you are going to use the iron method to PCB, print on to media the PCB image as is making sure the horizontal dimension to be 63mm.

Download the compressed 7zip file containing all the images and the PCB file in TIFF format.

Follow the illustrated images to place the components on PCB. It's so easy!

It is a nice circuit to play with and learn, quite versatile and practical as it can be used in many applications.

Have fun!