Circuit puzzle finding input resistance.

[@WisdomFolly]

1/T= 1/R3+1/R2+1/R1 The total resistance will always be less than the lowest resistor in the circuit. Brown Black Red = 1 Kohm

[@fifaham]

Exactly true, thank you for your input.

[@RayStill-i3u]

Aren't those 1,000 ohm resistors?

[@fifaham]

Absolutely, Left to Right : Brown - Black - Read - Gold >>> 1000 @5% tolerance. You can easily scale it to 100 ohms or 1 ohm, or even to 10,000 ohms. If R1 = 100 Ohm then Rin is about 33.3 ohms, if R1 = 1000 then Rin is about 333 ohms. Scale it as you wish.