Timer-switch circuit: two independent lighting channels
A timer switch operates lighting at scheduled times. Two channels allow two lighting groups to be controlled independently. This diagram explains the relationship between the supply, switching contacts and lamps. You can test both outputs manually in the interactive model.
Electrical safety: scope, competence and precautions
Put electrical safety first.
- These explanations and simplified diagrams support learning. They are not complete installation designs, site-specific risk assessments or a substitute for competent professional assessment.
- Electrical work can be life-threatening. It requires the competence and authorisation appropriate to the actual task, installation and jurisdiction.
- Errors and omissions can occur. Check the actual equipment documentation, applicable requirements and the complete installation before relying on an example; a working device is not proof of electrical safety.
- Personal protective equipment must suit the assessed hazards and task. It does not replace isolation, appropriate tools, competence or a safe system of work.
- Do not work on exposed live parts on the basis of these guides. Identify every relevant source, isolate it, secure against reconnection and prove dead using suitable equipment and procedures before access.
- Responsibilities for design, installation and verification must be established for the actual work. A website statement does not determine liability or remove duties under applicable law.
- This summary is not exhaustive. Where the circuit, condition or protective arrangement is uncertain, obtain suitable professional assistance before work proceeds.
- Complete the required inspection, testing and documentation before use, and maintain the installation and equipment appropriately.
Electrical terms explained
Electric current
The rate of flow of electric charge, measured in amperes (A). Conventional current direction follows positive charge flow, which is opposite to electron drift in a metallic conductor.
Voltage
The electric potential difference between two points, measured in volts (V). Always identify the reference points, such as line-to-neutral or line-to-earth; a voltage is not a property of one isolated terminal alone.
Resistance
The opposition associated with resistive current flow, measured in ohms (Ω). For a component behaving as an ohmic resistance under the stated conditions, U = IR.
Electrical power
The rate of electrical energy transfer, measured in watts (W). P = UI applies to steady DC; in a sinusoidal single-phase AC circuit, active power is UI cos φ using RMS values. The general average depends on voltage and current waveforms.
Ohm's law
For an ohmic resistance, current is proportional to the applied voltage: I = U/R. Resistance can vary with temperature, and non-linear devices do not necessarily obey one constant-resistance relationship.
Kirchhoff's laws
The current law expresses charge conservation at a circuit node. The voltage law states that the algebraic sum of voltages around a closed loop is zero within the applicable circuit model; induced effects must be represented appropriately.
Direct current (DC)
Current that maintains one direction. Its magnitude may be constant or vary; pulsating DC is still distinguished from current that reverses direction.
Alternating current (AC)
Current whose direction reverses with time, commonly periodically. Public supplies normally use approximately sinusoidal AC, while electronic equipment can produce other waveforms.
Frequency
The number of cycles per second of a periodic quantity, measured in hertz (Hz). Many European public supplies operate at a nominal 50 Hz.
Phase difference
The relative position of periodic quantities of the same frequency, often expressed in degrees or radians. For sinusoidal quantities, a time displacement can be related to that phase angle.
Circuit diagram
A drawing representing circuit components and their electrical connections using symbols. It is distinct from a physical layout or a complete installation design.
Capacitor
A component that stores energy in an electric field. Its capacitance relates stored charge to voltage under the stated conditions; a practical capacitor also has voltage, current and other operating limits.
Strappers in lighting circuits
The pair of conductors linking the changeover contacts in conventional two-way lighting switching, including intermediate switching where used. They carry the alternative switching paths and are not transformer windings; identify their actual functions from the circuit.
Coil or winding
A conductor arranged in turns, commonly to produce magnetic effects. Its behaviour includes inductance as well as the resistance and other properties of the actual winding.
Inductance
The relationship between flux linkage and current in the applicable model, measured in henries (H). Changing current produces an induced voltage; for a constant inductance its magnitude relates to L di/dt with the appropriate sign convention.
Capacitance
The relationship between stored charge and voltage, measured in farads (F). For a constant capacitance, changing voltage produces current according to i = C dv/dt.
Transistor
A semiconductor device used for switching, amplification or other control functions. Its operating principle and control quantities depend on whether it is, for example, a bipolar transistor or a field-effect device.
Semiconductor
A material whose electrical behaviour can be controlled through composition, electric fields, light or other conditions. Semiconductor devices use this behaviour for functions such as rectification, switching and amplification.
Diode
A device with a strongly directional current-voltage characteristic. Real diodes have forward losses, reverse leakage and limits; they do not act as perfect one-way valves at every voltage.
Relay
A device that changes an output in response to a control input. An electromechanical relay uses a coil and contacts, while a solid-state relay uses electronic switching elements.
Switch
A device that makes, breaks or changes electrical connections. Its rating, contact arrangement and permitted duty determine the applications for which it is suitable.
Electrical network
An interconnection of electrical components and branches. Series, parallel and more complex arrangements can be analysed with an appropriate circuit model.
Transformer
A device transferring energy between magnetically coupled windings through changing flux. Turns ratio influences voltage and current relationships; losses, limits and any conductive connection depend on its construction.
Electromagnet
A magnetic arrangement energised by electric current, commonly using a coil and magnetic core. Contactors, relays and actuators use electromagnetic force for mechanical operation.
Impedance
The relationship between voltage and current phasors in a sinusoidal steady-state model, measured in ohms. It combines resistance and reactance and can vary with frequency.
Short circuit
An unintended low-impedance connection between points at different potentials. Current is limited by the complete source and fault-path impedance, not literally unrestricted.
Electric circuit
An arrangement of sources, conductors and components. Current flows where a complete conductive path exists, although a circuit can also be described in an open or disconnected state.
Series connection
Components in one unbranched path carry the same current. Their voltages add algebraically; the individual voltage shares depend on their electrical behaviour.
Parallel connection
Branches connected between the same two nodes share the same voltage. The branch currents depend on their loads and add algebraically at the connection points.
Fuse
An overcurrent protective device with an element designed to melt and interrupt the circuit under specified conditions. Its rating, characteristic and breaking capacity must suit the application.
Overcurrent protection
Protection against overload and short-circuit effects, commonly using fuses or circuit-breakers. A sensing relay may require a separate interrupting device; an ordinary RCCB does not provide this protection by itself.
Earthing
A deliberate connection of a system point or conductive part to earth. Protective earthing forms part of an electric-shock protection arrangement; it must not be confused with neutral, functional earth or a guarantee of zero touch voltage.
Frequency converter or variable-speed drive
Electronic equipment that produces a controlled output frequency, commonly with controlled voltage, to operate a suitable motor. Its pulsed output, protection and compatibility requirements differ from a direct mains supply.
Electromagnetic compatibility (EMC)
The ability of equipment to operate satisfactorily in its electromagnetic environment without causing unacceptable disturbance to other equipment. Both emissions and immunity matter.
Power loss
Energy dissipated per unit time, commonly as heat in conductors, cores, insulation or semiconductor devices. Ideal inductors and capacitors exchange stored energy without average loss; practical components have additional loss mechanisms.
Short-circuit current
The current resulting from a short circuit. Prospective fault current is assessed to select protective devices with adequate interrupting capability and to check thermal and mechanical effects.
Power factor
The ratio of active power to apparent power. For sinusoidal voltage and current it equals cos φ; with distortion it also reflects waveform effects. It is not the same as energy-conversion efficiency.
Insulation resistance
Resistance measured between specified conductive parts separated by insulation under stated test conditions. The test voltage, connected equipment and interpretation must suit the circuit; it is not a complete assessment of insulation performance on its own.
Construction and operation
Timer K1 is supplied through L and N. Each of its two switching contacts also receives a separate L feed from the same upper supply rail. Channel 1 controls lamp 1; channel 2 controls lamp 2. Supplying the timer and supplying the switching contacts serve different purposes: a working display alone does not mean that the contact side can supply a lamp.
In a real device, a time programme determines when a channel switches on or off. A daily programme repeats the same times each day; a weekly programme can distinguish between weekdays. Available modes depend on the device. For the equipment itself, see the timer switch as a switching device.
Circuit diagram with two independent channels
Reading the diagram
The three equal-length upper rails represent L, N and PE. The separate L feeds to the channels come from the same supply; they do not represent different line conductors. A closed contact connects its L feed to output L′ 1 or L′ 2. The switched conductor then runs to the corresponding lamp. The lamps' N and PE connections are not switched in this diagram.
Red highlights a path connected to L, including the part leading up to an open contact. It does not mean that operating current already flows through that open branch. Dark blue highlights the corresponding neutral connection: the timer supply remains dark blue, while a lamp's N branch turns dark blue when that lamp is on. The neutral connection still exists when the lamp is off. These colours are learning aids, not a test for absence of voltage.
The PE connection at K1 represents an enclosure connection for a design that requires it. It is not a universal terminal on every timer switch. Devices of other protection classes can differ; the manufacturer's example in the sources is class II. L′ 1 and L′ 2 are functional labels here, not transferable terminal numbers.
Manual test and clock display
Use “Channel 1” and “Channel 2” to switch each output on or off. The selected state remains after you release the button. The display uses your browser device's clock to show Central European time; the label changes between UTC+1 and UTC+2 for standard time and summer time. The clock does not trigger automatic switching in this model. No time programme or countdown is simulated.
“Static diagram” opens the ordinary drawing. It is also available without JavaScript and can be enlarged. Pausing the lamp glow effect does not stop the clock.
Supply and wiring principle
The timer needs its specified supply, and each contact needs the connection appropriate to the application. Whether the contacts are volt-free, which terminal is common and which loads are permitted must be established from the actual device documentation. This functional diagram does not specify protective devices, conductor sizes or a particular terminal arrangement.
Assess the switching capability against the actual load. With LED lighting in particular, inrush current and the permitted lamp load can affect device selection. A general current rating alone is not universal approval for every lighting group.
This example explains a fixed installation. Connection, selection of protective measures and verification require appropriate electrical competence. Follow the requirements applicable to the installation and the manufacturer's instructions.
Functional test: all four channel combinations
Operate channel 1 alone, then channel 2 alone, and finally both together. Each output must affect only its own lamp in the model. Switch to the static view and back: the previously selected interactive states are retained.
| Channel 1 | Channel 2 | Lamp 1 | Lamp 2 |
|---|---|---|---|
| OFF | OFF | off | off |
| ON | OFF | on | off |
| OFF | ON | off | on |
| ON | ON | on | on |
For a real installation, also check the date, time, weekdays, time programme and any active manual or permanent override. A successful manual test does not replace verification of the time programme or electrical verification of the installation.
When is this lighting circuit useful?
Scheduled lighting suits lighting groups needed at known times. Two channels allow different operating times without switching both groups together. Programmes need to follow changes in use; time alone does not tell the timer whether anyone is present or whether lighting is needed.
A staircase lighting circuit instead starts with a push-button and turns off after a set duration. In an impulse-relay lighting circuit, each new push-button impulse changes the maintained state. A two-gang lighting circuit directly controls two groups from one position. The key distinction is whether clock time, an elapsed duration or a user action determines the state.
Questions about timer-switch circuits
Why does each channel have its own L feed here?
The model separates the timer supply from the two contact feeds. Both channels receive L from the same rail. The connection diagram of the actual device determines which links or feeds are required.
Are the lamps connected in series?
No. Each lamp has its own switched line path and a connection to N. Turning off one channel does not interrupt the other lamp's operating path.
Why does the running clock not switch the model automatically?
The clock display and manual test buttons are separate functions. This lets you explore the contacts without waiting for a scheduled event. Automatic on and off times are programmed in a real timer.
The clock works but the lamp stays off. What could cause this?
Possible causes include an inactive time programme, a different channel state, a missing contact feed or a fault in the lamp branch. The display alone does not identify the cause. A real installation requires appropriate testing of the actual equipment.
Sources and scope
Manufacturer's example of a two-channel timer: Theben TR 612 top3 product data. The manufacturer's instructions dated 20 March 2025, in German cover connection, programming and manual operation. This generic model does not reproduce that product's terminals or user interface.