I'm doing a replication of vehicle hi/low beam switch in my "lab" and I'm not sure what's the right type of switch for it. It has to have a dedicated on/off switch (the one on the bench power supply doesn't count) so that should be a basic SPST on/off switch that makes or breaks the circuit when PSU is on.

However, there are 2 lights (LEDs) here, one hi beam, one low, and low beam needs to be on by default, and high beam additionally turned on when needed. What is the correct type of switch for this?

The pushbutton is there to flash the light #2.

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1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. A circuit suitable for demonstration. A single-pole, double-throw (SPDT) switch is required.

In a real car the headlamps are typically 55 W on filament lamps and the circuit would have to switch more that 10 A into cold filaments. For this reason the switches control relays which do the hard work.

I misread the requirement for always on DIPs.


simulate this circuit

Figure 2. A single-pole, single-throw (SPST) switch will suffice.

  • \$\begingroup\$ Am I reading this wrong, but when the High switch is on, the Dip light is out of the circuit? \$\endgroup\$
    – Varonne
    Commented Feb 23, 2021 at 9:29
  • \$\begingroup\$ You're right. I misread the requirements. See the update. \$\endgroup\$
    – Transistor
    Commented Feb 23, 2021 at 10:18
  • \$\begingroup\$ No problem, I appreciate the help, yeah it's much simpler than I thought, I'm a newb though :D so, may I ask a stupid question #2 - the circuit you draw has 3 grounds drawn, but it could be drawn with a line connecting the battery ground to the both light source grounds? Would that be more of a standardized way to draw it? And another dumb question, are they in parallel or series? \$\endgroup\$
    – Varonne
    Commented Feb 23, 2021 at 10:57
  • \$\begingroup\$ All grounds may be replaced by a connecting wire. The ground symbols are nice because they immediately tell you that they're the reference point, they're 0 V and they're all connected. In the case of a car the chassis metal is used as the common return conductor. This is the standard drawing method. e.g. public.fotki.com/Lordmodelbuilder/vanning/van_pictures/…. The bulbs are in parallel. If they were in series they would dim when both were on. \$\endgroup\$
    – Transistor
    Commented Feb 23, 2021 at 11:24
  • \$\begingroup\$ Pardon my ignorance, but is the difference from "series" and "parallel" in the grounding? If I wanted to connect these in series, bulb A's negative terminal would connect to bulb B's positive? So parallel would mean any circuit where each device draws power for itself separately? \$\endgroup\$
    – Varonne
    Commented Feb 23, 2021 at 13:26

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