# How do I build a circuit to dim an AC powered lamp at low light levels

Joined Apr 5, 2024
3
I have a college group project to build a traffic light using only analog components (no microcontrollers ), my subsection is to design and build the driver unit and a circuit that dims a set of ac lamps by 60% at night. I receive digital signals from a 555 timing circuit. Now, I've managed to find resources to build the actual driver unit to turn the lamps on using the signal, and resources to build a circuit with the LDR, but cannot find a way to merge my 2 circuits together so that the lamp dims when ambient light is gone. The circuit also has to account for hysteresis so I thought of using an op-amp as a Schmitt trigger, but I cannot figure out how to incorporate it into the AC dimmer circuit either. Attached are screenshots of the 2 separated circuits which work fine separately. Oh, also, for the LDR circuit, it's a modification of a regular AC dimmer circuit but I attempted to put the LDR in place of a variable resistor, and my idea was to have the LDR's resistance be the required resistance to dim the circuit. I also haven't figured out the value of this YET, as I wanted to first get a circuit that actually works and dims and thereafter figure out the appropriate value.

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#### ericgibbs

Joined Jan 29, 2010
19,017
Hi D3A,
I assume your U(2A) signal is from a 555 a phase zero crossing circuit?

If so, have you considered using the LDR to modify the 555 output so that it is PWM signal, based on the light intensity on the LDR?

E

#### Ian0

Joined Aug 7, 2020
10,067
Traffic lights that still use filament lamps? They are all LED now.

If you want to reduce the brightness of an AC filament lamp by ~60%, put a diode in series (and hope the circuit will not have to pass harmonic current regulations). The voltage is reduced to 1/√2, so the brightness will be reduced to (1/√2)^3, which is 35%, so it is reduced by 65%.
The only problem is that your lamps will be behind colour filters, and the colour will be shifted towards the red by dimming it, so the green will be much more dimmed than the red.

#### MrAl

Joined Jun 17, 2014
11,583
I have a college group project to build a traffic light using only analog components (no microcontrollers ), my subsection is to design and build the driver unit and a circuit that dims a set of ac lamps by 60% at night. I receive digital signals from a 555 timing circuit. Now, I've managed to find resources to build the actual driver unit to turn the lamps on using the signal, and resources to build a circuit with the LDR, but cannot find a way to merge my 2 circuits together so that the lamp dims when ambient light is gone. The circuit also has to account for hysteresis so I thought of using an op-amp as a Schmitt trigger, but I cannot figure out how to incorporate it into the AC dimmer circuit either. Attached are screenshots of the 2 separated circuits which work fine separately. Oh, also, for the LDR circuit, it's a modification of a regular AC dimmer circuit but I attempted to put the LDR in place of a variable resistor, and my idea was to have the LDR's resistance be the required resistance to dim the circuit. I also haven't figured out the value of this YET, as I wanted to first get a circuit that actually works and dims and thereafter figure out the appropriate value.

View attachment 319201
Hello,

The first thing to think about is if you want to use a triac opto coupler with a zero crossing detector or without a zero crossing detector.
A opto with a zero crossing detector is typically used in a solid state relay. An opto coupler without a zero crossing detector is typically used for phase controlled dimming. The reason for this is because dimming usually requires phase control while an SSR does not need that it just needs to switch 'on' with low RF radiation which means it will only switch 'on' near the zero crossing.
That's not to say you can not use an averaging method over several cycles of the line frequency, but for a traffic light I would think that approach would be highly objectionable due to the required on/off cycle times. It's even possible that could trigger someone with epilepsy to have a seizure.

The second thing to think about is the relationship between the sinusoidal duty cycle versus the lamp intensity. You have to establish the relationship there in order to be able to decide on a duty cycle, which I assume for now would be fixed. Note this duty cycle will be different than what you would see with a regular rectangular wave because the available power various over the set duty cycle AND the sinusoidal phase with a sine wave, while with the square wave it only varies with the set duty cycle.
This can be calculated using the average power formula, but ideally the relationship between the applied power and lamp intensity also becomes a factor because the filament runs cooler as the intensity is forced to a lower level. This means you have two nonlinear relationships to establish. You can calculate one, but probably have to look up the relationship between intensity and power input for an incandescent lamp. You may also have to consider the color spectrum, but this assignment may not be that in-depth.

In short, the LDR could be used with a comparator, the comparator triggers the triac pulse generation. The pulse generation must be synced with the line zero crossing, but this zero crossing detection works differently than the opto zero crossing detection as it only tells the pulse generator when to generate a turn-on pulse. Then, the pulse must end before the start of the next half cycle.

Missing from your diagrams so far is the pulse generator circuit, and it looks like the 'zero crossing' triac opto coupler is an error it should be the type that does not trigger only on the zero crossing.

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