AC Light Dimmer Module Circuit

AC Light Dimmer Module with ZC Detector

From time to time I get requests to make some high power switching, so I decided to design a digital dimmer module that could handle a lot of different situations. The original requirements laid down that the design must be of the open type, that is, it must be possible for individual users to develop a program or alter an existing one for controlling an ac load through the dimmer module.

The compact ac light dimmer module with zero-crossing detector described here is ideal for microcontroller-based ac voltage controlling applications and projects. This module contains a triac triggering coupled with zero-crossing signal detection mechanism for programming the intensity of incandescent lamps and/or fan speed controlled through a microcontroller circuitry. Circuit of the module shown here offers improved performance and reliableness using very little power and just a few plain vanilla components.

The AC Light Dimmer Module Circuit

It is clear from the circuit diagram that the key component of the design is a standard triac (TR1). The circuitry which is linked to the microcontroller via the signal input header (H1) drives an optoisolator triac driver (OC1). One set of terminals of this component is connected to the triac, whereas the set of terminals at the otherside is switched via the I/O line of the external microcontroller and an integral current-limiting resistor (R1). Since the triac incorporate a snubber network, the dimmer can be used for controlling inductive loads,too. The next optocoupler (OC2) serves to detect the mains zero-crossing.

Zero-crossing is used for synchronizing the dimmer. The optoisolator is linked directly to the mains supply since a transformer causes an unwanted small phase shift that may induce anomalies in the performance. The synchronizing pulse is buffered by a small signal transistor (T1) and routed to the signal output header (H2).

The circuit can be assembled on a small perfboard, and the work is straight forward as long as the specified components are used. Since several tracks carry the full mains voltage, extreme care is required in the assembly. Always unplug the module from the mains before doing any work or checking some thing after assembly. Also note that to ensure correct operation the module should be connected to a frequency-stable mains supply only. The dimmer module may be used for stage lighting, for controlling domestic lights, for illuminating aquariums, or for mood lighting.

AC Light Dimmer Module Circuit

Since the switching input (SW) and zero‐crossing (ZC) output are available on separate headers (H1 and H2) on the board, this module is still usable without a microcontroller. Further, you can add or remove components based on particular needs. For example, incase no need for inductive load control just omit the snubber (C3-R5). On the other hand, if you want to integrate the module into an existing digital design, just skip the “external” microcontroller idea and use the switching input and zero-crossing output connections directly.

Parts List

  • R1,R4=180R ¼ w, 180R 1w
  • R2,R3=680R ¼ w, 680R 1w
  • R5=39R 2w
  • R6=56K 1w
  • R7=10K ¼ w
  • R8=1M ¼ w


  • C1=100nF
  • C2=100nF 275V ~
  • C3=10nF 630V~


  • OC1=MOC3021
  • OC2=4N35
  • TR1=BT136 mounted on heatsink
  • T1=BC547B
  • D1=1N4001
  • LED1= 3mm Red 10mA


  • H1, H2 =3-pin male headers J1,J2=PCB Screw Terminals…

Application Hint

As you may noticed, the module comprises two independent input and output connections; one pair in the low-voltage side (H1 & H2) and other in the high-voltage side (J1 & J2). You can feed the “dimming control signal” output (SW) from an I/O port of a microcontroller to the input header H1, and the “zerocrossing signal” output (ZC) available from the module (through output header H2) to an I/O port of the same microcontroller. Similarly, connect the utility power at AC-IN connector (J1) and intended control device into AC-OUT connector (J2). An indicator (LED1) is provided onboard to test the triac trigger system. This indicator will be full bright when LOAD is 100% ON state. The triac in the module BT136 supports 4A Max. i.e 920W @ 230VAC. Incase of a higher-current requirement, you can change this triac with a suitable type. The heatsink would still be required for the triac to safely handle higher currents (assuming operation at room temperature).

zc module connections

Note: Network C3-R5 is for snubbing of the triac (TR1), and C2-R3 for snubbing of the coupler (OC1). These components may or may not be necessary depending upon the particular triac used and the load connected.

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