Eye Candy 5

A Simple 555 Led Throbber

This is a simple throbber designed for use with a blue or white LED, using a minimum number of parts but no tweakability apart from speed.

It uses a 555 astable oscillator with the 'On' and 'Off' times approximately equal. The voltage on the timing capacitor C1 is followed by a current-boosting transistor Q1 connected as an emitter-follower. To minimise the effect of "sampling" on the capacitor voltage I've used a little NPN Darlington transistor, the BC517. With a DC gain typically around 60,000 its base current, drawn from the R1-VR1-C1 timing components, is under 100 nano-amps, well below the normal leakage current of C1. There are several alternatives – MPSA13, MPSA14, ZTX601, ZTX605 for example – but check the pin-out on the maker's datasheet before fitting.

The 555 timing capacitor always cycles between ¹/₃ and ²/₃ supply voltage, 4V and 8V on a 12V supply, and the darlington emitter-follower then loses 1.2V between base and emitter, leaving the LED and its resistor cycling over a 2.8V–6.8V range, so blue and white LEDs throb nicely.

The pre-set variable resistor VR1 alters the throb frequency, with R1 limiting the current at the minimum setting. Resistor R2 limits the maximum LED current to about 4mA which I found gave a better dimming cycle than using higher values. (Although LED specifications frequently quote forward currents of around 20mA, the difference in brightness at much lower currents is only slight, giving a poor start to attempts to dim the LED.)

The 100nF decoupling capacitor C3, connected across the 555 supply pins close up to the chip, I found essential. The BC517 is so sensitive it was picking up the 555 switching noise and producing an unwanted flicker from the LED at the darkest point in the cycle.

The sawtooth waveform from the 555 timing capacitor is not as linear as the triangle-wave from the two op-amp circuit, but by restricting the exponential charge/discharge curves to their middle portions (as the 555 does) it's OK for the job.

Frequency is 1/(1.4 x [R1 + VR1] x C1)

Construction

Construction Guide – Component side view

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
1		LED(a)													R2
2					j2									j5		j8
3		LED(c)		j1			C1(-)	C3	U1#1	(br)		U1#8				j8
4						j4	C1(+)		U1#2	(br)		U1#7	(br)	j5		Q1(c)
5							R1		U1#3	(br)		U1#6		j6			Q1(b)
6		12V			j2			C3	U1#4	(br)		U1#5	C2	(br)	R2	Q1(e)
7				j1	j3								C2
8		0V			j3		(br)	VR1							j7
9							R1					VR1w			j7
10						j4		VR1						j6
11

The circuit is shown on an 11 strips x 18 holes piece of stripboard, with 7 track breaks as shown in red (4 between the IC legs). Breaks can be made with a few turns of a hand-held drill bit if you don't have the proper tool.

Start by marking out where the IC is going, then you can "dry" fit the links and other components to check fits.

For miniature pre-set VR1 there's room for a variety of footprints, single- or multi-turn, and there's room in the left-hand columns for 0.2" pitch PCB connectors to attach power and LED wires.

Start soldering with the lowest components first (the wire links) so you can lay the board down with them pressed against the surface to solder. Move on to the resistors, the 555, small capacitors and transistor, finishing up with the trim-pot, tall capacitor and PCB connectors (if used).

Clip off excess component leads as you go. Finally check for faults or solder bridges between the tracks and test.

I used an aluminium electrolytic capacitor for C1, it does need to be low-leakage for the low frequencies, so a tantalum-bead type might be a better (but more expensive) choice. The voltage isn't critical, 25V or 50V ratings should still fit OK. Just make sure it's the right way round, positive terminal as shown.

Although designed for blue or white LEDs with a minimum forward voltage around 3V, the circuit can be used with "2V" LEDs in red, amber, green, etc, if a pair of ordinary diodes (eg, 1N4148, 1N4001, etc) are placed in series with the LED to bring the overall forward voltage up to match.