Pentax Spotmatic: What is the Bridge Circuit?

A voltage-independent Wheatstone circuit

By FozzFoster in Articles and Tips on Nov 30, 2021
Pentax Spotmatic: What is the Bridge Circuit?

Most of the Pentax Spotmatic cameras were designed to use a 1.35V PX-400 mercury battery to power their internal light meters. However, mercury was banned in batteries in 1996. So, what batteries can be used instead?

Fortunately, the Spotmatics (SP, SPII, SPIIa, SP500, SP1000) are designed with a balanced bridge circuit: This allows a photographer to use the readily available 1.55V silver-oxide S400PX batteries, regardless of that fact that they supply a higher voltage!

The bridge circuit in these cameras are relatively immune to changes in voltage and will still provide accurate light meter readings. Please enjoy this PentaxTips video below for the full details:

Hungry for more?  Check out our previous video on how to spot the legendary 8-element Super Takumar lens.

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