Interpret the circuit for a Carbon Monoxide detector

[Rtg]

I am trying to build a CO detector and have TGS2442. How can I implement the basic circuit given in the datasheet page 2?
http://html.alldatasheet.com/html-pdf/144246/ETC1/TGS2442/198/2/TGS2442.html

I cannot understand how Vc and Vh can be applied together and the functioning of the BJTs given.

 

[Alec_t]

With the circuit shown you would have 5V as the +ve rail. For the heating cycle you would turn on the top (PNP) transistor for 14ms every 1 sec. Due to the Vce voltage drop there would be 4.8V across the heater (terminals 1,4). For reading the sensor output you would turn on the lower (NPN) transistor for 5ms every 1 sec and take the measurement half way through that 5ms period. From the waveforms shown, the PNP is turned on immediately after the NPN is turned off.