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I have difficulties to understand how the below circuit can detect radar waves by only using C1 and the lengths of its wires to IC1. Can this work? And if yes, how? What is the physics theory behind this?

This circuit uses a 1458 dual op-amp to form a radar detector. C1 is the detector of the radar signal. The first op-amp forms a current-to-voltage converter and the second op-amp buffers the output to drive the piezo transducer. R5 sets the switching threshold of the second op-amp; normally it is adjusted so that the circuit barely triggers on background noise, then it's backed off a bit. The response of the circuit may be tuned by adjusting the length of the leads on C1. For typical road-radar systems, the input capacitor's leads should be about 0.5 to 0.6 inches long.

source: "Radio-Electronics" Magazine, Jul,86 issue (C) Copyright Gernsback Publications, Inc., 1986

("Radio-Electronics" Magazine, Jul,86 issue (C) Copyright Gernsback Publications, Inc., 1986)

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  • \$\begingroup\$ Odd that it uses a 9V supply, rather than the 12V readily available in a car. \$\endgroup\$ Jan 15 '18 at 10:33
  • \$\begingroup\$ Car power is dirty as hell. For sensitive circuits running of a seperate battery is probablly easier than taking power from the car. \$\endgroup\$ Jan 15 '18 at 16:52
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    \$\begingroup\$ @user3190797: Due to some states having laws allowing police to confiscate any radar detecting "device", you probably want it completely physically isolated from anything else you care about so that there's no question of the scope of what they can confiscate... \$\endgroup\$ Jan 15 '18 at 20:43
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What is the physics theory behind this?

This circuit supposedly detects radar speed traps passively. Just in case there is any confusion, it isn't part of a radar system - it's a detector that can sense the presense of (say) hand-held speed guns used by some police forces.

It "works" because at high frequencies the LM1458 op-amp is quite susceptible to EMI and this gets rectified at the input stages and a dc offset is created. It's a bit flakey in performance and was aimed at the DIYer so it isn't really anything reliable but I suppose some were made back then and may have delivered functionality.

The length of the tracks/wires that is formed in the C1-loop forms a resonant tuned circuit supposedly. Links to this circuit can be uncovered by googling "1458 radar detector" and there does appear to be a youtube video.

Here's an article from Analog Devices (MT-096 RFI Rectification Concepts) that explains the phenomena.

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You resonant the capacitor leads with the ONCHIP capacitance (ESD structures + input diffpair parasitics). That large capacitor is just a place-holder for the error voltage generated by rectification on the junctions.

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