Solid State Nuclear Track Detector (SSNTD) based diffusion chambers have been widely used for residential radon measurements due to their cost effectiveness, portability and easy-to-use feature. In India, an LR-115 track detector based twin-cup dosimeter has been in use for about a decade for indoor ²²²Rn and ²²⁰Rn measurements. However, the estimation of the gas concentrations using this dosimeter was based on the assumption of the same entry rate of the gases into the two cups of the dosimeter, which may not be valid for dosimeters deployed in turbulent environmental conditions. To overcome this limitation, a new pin-hole based ²²²Rn/²²⁰Rn discriminating measurement device has been developed. The underlying discrimination technique has been established by modelling ²²²Rn and ²²⁰Rn diffusion into a pin-hole chamber and validating the same by carrying out experiments in a test chamber. The device has been calibrated at Bhabha Atomic Research Centre, Mumbai following the standard procedures to correlate the number of tracks registered in the LR-115 detector placed in the two chambers to the ²²²Rn and ²²⁰Rn concentration in the environment. Salient features of the device include (i) the pinholes act as ²²²Rn/²²⁰Rn discriminator and eliminate the requirement of membrane filter used in the earlier twin cup design (ii) the single entrance design for gas transmission and (iii) use of multiple pinholes of reasonably small radius minimises effect of turbulence on ²²²Rn/²²⁰Rn transmission factors so that the calibration factor is independent of indoor turbulence.