Do you have an airport or seashore on your layout? If so, you’ll want to rush to your bench and build this circuit. It provides a lamp which blinks slowly on and off (more off than on) and the light intensity builds slowly from off to on and off again — just like the rotating beacons at airports or seaside lighthouses.
This circuit is the original work of John Decker, and appeared in MR over 20 years ago.
The circuit is very forgiving, and junkbox substitutions may be made for most parts. Everything but the actual bulb is available from Radio Shack and others.
CIRCUIT HINTS & TIPS
1. Resistors R2 and R3 set the blink rate of the lamp; increasing the value of R2 will increase the “ON” time of the lamp whereas increasing R3 will lengthen “OFF” time. Experiment to suit yourself.
2. Resistor R5 determines the rate of increase of intensity, where R6 and R7 determine the rate of decrease. Again, experiment to suit yourself, but maintain approximately the same ratio between the values of R6 and R7 (ie, 4:1).
3. The potentiometer (R10) controls the brightness of the lamp. Ensure that the pot is set for maximum resistance when you first apply power. If the bulb is too dim, adjust R10 to suit. If you’re using a conventional pot (rather than a trimpot), make the connections while looking at the back of the pot (ie, shaft facing away from you, terminals at the top); attach the wire from the Q3 collector/R8 junction to the rightmost pot terminal; tie the center and leftmost terminals together, and thence to the base of Q4. When the pot is rotated fully counter-clockwise, resistance is maximal & brightness is minimal. For initial power-up, rotate the shaft fully CCW.
4. Power transistor Q4 needs a heat sink attached to ensure it doesn’t overheat.
5. Capacitor polarity can matter. Electrolytic capacitors (such as C1 & C4) are “polarized” — that is, they have a “plus” and a “minus” terminal. Make very sure you connect them as shown in the schematic diagram (otherwise, bad things happen). Capacitors C2 & C3 are “non-polarized” electrolytics; they don’t care which way they’re connected. If you have trouble finding 1uf non-polarized caps, you can substitute two 2.2uf/35V standard electrolytics in place of each non-polarized 1uf unit; simply connect the “plus” (+) terminals together (and to nothing else) and then the “minus” (-) terminals become the leads of the new capacitor.