Pentax Spotmatic: What is the Bridge Circuit?
A voltage-independent Wheatstone circuit
By FozzFoster in Articles and Tips on Nov 30, 2021
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.
More from the Pentax Forums Homepage
- Announcing Our December, 2023 Photo Contest
- Pentax eBook Sale: Save 25%
- October 2023 "Night Scene" Photo...
- Black Friday: The Ultimate Forum Account Upgrade!
- The New "Best of Pentax Forums"...
- Pentax WG-90 point and shoot camera announced
- Now Live: November 2023 Limited Lens Giveaway
- October "Night Scene" Contest...