Component substitutions

joshkz sent me a private message about substituting components.

All resistors: higher power types (1/2W, 1W, etc) can be substituted with no change in performance. If you are using a socket board, anything larger than 1/2W won't fit (leads are too fat), and even the 1/2 watt leads probably will distort the internal socket contact. Not a problem unless you want to insert a thinner wire or lead in the same location at a later time. Trimming the ends of the leads with sharp wire cutters so they come to a point will make them easier to insert in the breadboard.

Here are my thoughts on substituting alternate resistor values. Reference designations are from the 2nd generation Clark schematic.

R1 - 1.0K timing resistor. Can be increased as long as R2 and C2 are changed by the same proportion. For example, if you have only larger value resistors available, R1 can be INcreased to 10K (a 10-to-1 change) ***IF*** R2 is INcreased from 3.9K to 39K ***AND IF*** C2 is DEcreased from .0047uF to 470pF. By the way, making these changes will increase battery life.

R2 - 3.9K timing resistor. See R1. If you are increasing this value, best to keep it below 100K.

R3 - 1.0K output resistor. Best not to change. Larger values decrease the current flow through the body, smaller ones increase it to possibly dangerous levels.

R4 - 3.9K LED resistor. Can be changed, especially increased, within reason with no change in circuit performance. Larger values make the LED dimmer and increase battery life. Smaller values make the LED brighter but suck up more juice. Do not decrease below 150 ohms. This sets the average LED current to about 20mA, the recommended operating level for most generic LED's.

R5 - 39K offset resistor. Can be increased with little change in circuit performance. The original circuit did not have this part (effectively an infinite value), so anything between 39K and 1.0Meg is a compromise between the 1st and 2nd gen designs. Values smaller than 39K increase the offset (no known benefit), and decrease the peak-to-peak output voltage swing (probably bad).

As for the capacitors:

C1 - control voltage noise filter. The 555 will operate just fine without this part, but the output pulse width will be a bit more stable with it in place. .01uF is the minimum value recommended by the original chip inventor. Increasing to just about anything will have no effect on circuit performance. Decreasing it will have no affect unless you have extreme levels of electrical noise around you. For example, making it the same size as timing capacitor C2 is perfectly acceptable.

C2 - timing capacitor. Leave this value alone, unless you are changing the values of all 3 timing components (C2, R1, R2) in the correct proportions. See R1 above.

If you want to muck around with the timing components to achieve different operating frequencies or duty cycles, there are graphs and simple equations on the 555 data sheet. Also, there are a ton of websites with 555 calculators.

ak