Right - you are talking about the screw on valve caps and yes they are MUCH cheaper than the valves that send a signal that you insert into your wheels as per the Tire Insight kit that I was offering you.
I have tried 4 different styles of the valve caps and all kits proved, lets say, 'less than satisfactory'. Partialy due to inaccurate pressure reading but more significantly inadequate communications meaning that I had to often reset the caps to regain contact.
Perhaps the latest versions are better but if you wish to try one type that I still have PM me and I can post a new set to you for the cost of the postage. They should should be able to communicate with your head as they should be ok with an Android phone app . However they have their own display unit that you can suction fit to the dash or screen and as long as it can see the sun will keep itself charged by way of the integrated
solar panel. They have been tried on my car but the kit is in effect new.