How to slow the charging and discharging of a 555 timer without changing the frequency.

Question

Imagine driving at night and seeing a radio tower in the distance. At the top, a red light appears to brighten and dim instead of just blinking on and off. How can this effect be accomplished with a 555 timer? I assume the charging and discharging times must be increased, but I don't want to change the on/off frequency of around 1hz.

jc2048 · Accepted Answer

So, quickly hacking together something derived from the sort of things suggested by John and Doug, would this do what you want? Needs a large electrolytic value to get the timing, though, and not really a 'good' circuit [far too finicky], but it's interesting to play with. By the way, it's a standard red led: the intensity, relative to the rest of the scene, overdrives the camera sensor and just makes it look white.

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fmilburn · Answer

It sounds like you want to create something like a 1 second sine wave. A search on the internet for "555 timer sine wave" will turn up a number of hits. Edit: it requires a filter and won't be a perfect sine wave of course

shabaz · Answer

Lots of ways to do this : ) but if you wish to stick with using 555's for this, yet another approach is to place a 4017 on the output, and it will generate 10 outputs, pulsed in sequence (you'll have to increase the 555 frequency from 1 Hz to 10 Hz, but the final output frequency from the overall circuit will still be 1 Hz). Then, by arranging 10 resistors as the top parts of a potential divider, and an eleventh resistor as the bottom part of a potential divider, you've got very near to a sinewave. (Use Excel to calculate the resistor values). A final output resistor and capacitor filter will perfect it. You'll need an op amp or similar as John mentions, to drive the LED, there won't be sufficient current from the filtered output to drive the LED directly. You could also consider PWM, that's a bit more advanced. Some handy parts for 555 experiments: ICM7555 (sometimes better than NE555) CD4017 or CD74HC4017 - counter with 10 outputs LM2901 - comparator MCP601 - op amp BC547 - NPN transistor BC557 - PNP transistor BSS138 - N-channel MOSFET BSS84 - P-channel MOSFET (Both of the MOSFETs are surface-mount but large enough to attach wire strands if desired, or solder onto a proto board adapter. Too hard trying to work around not using surface-mount, since it's the current reality that most parts are now surface mount). With parts from the above list, and resistors and capacitors, you should be able to come up with several different ways to implement your design.

jw0752 · Answer

Hi Roger,

Try putting a 1000 uF capacitor across the load on the output of the 555. You should also add an in series resistor to the output. Keep in mind if the light that you are trying to fade in and out is an LED it is sometimes difficult to see the effect of a change in voltage. You can experiment with the size of the capacitor and the in series resistor. To get a more powerful output than the 555 can deliver you can use the output of the 555 modified with the previously described resistor capacitor to drive an OP Amp and a driver transistor of some sort.

John

dougw · Answer

Probably the best waveform to use would be a triangle wave. 555's are not great at making triangle waves and there are simple op-amp circuits you can google, but if you have a 555 generating a square wave at 1 Hz, you could feed that into an op-amp integrator circuit to turn it into a triangle wave. If you find it simpler, you could also amplify the capacitor signal of the 555 (which is a triangle wave with curved slopes) using an op-amp.

How to slow the charging and discharging of a 555 timer without changing the frequency.

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