T1 and T2 are NPN,...
I added the inset, with the leads labelled, but the schematic's symbols remained unchanged.http://www.jameco.com/Jameco/Products/ProdDS/178511.pdf
Disconnect "the circuit" from the RPi.
If you put the input resistor to +5 then T1's collector should be Gnd/0V and T2's collector should be +5.
If you put the input resistor to Gnd then T1's collector should be +5 and T2's collector should be Gnd/0V.
If you have access to the RPi's "1.8V", do the same using that instead +5 for the input.
I hope we have the Grounds in common: the RPi's goes to the circuit's goes to the TTL device's. Please confirm.