Recreating this transmitter circuit, need advice - Page 1
Forum section: electronics / rf-microwave

author: MWisBest
author_rank: Contributor
timestamp: November 02, 2017, 02:06:01 am
content: I've got a pair of remote control cars but only have a single transmitter. Rather than buy a sketchy used one off of eBay I was hoping to create one from scratch. I found a schematic for it but I'm certain it could be improved, so I was hoping for some advice; I am completely RF-illiterate.The IC/encoder block appears to belooselybased on the Pericom PT8A995 or something like that, since the receiver circuit on the cars uses a Pericom PT8A201W. I don't think this will be much trouble to recreate on a more "standard" microcontroller.What I see beyond that on the schematic is...basic input mux stuff (like any remote control), plus steering angle and speed offset controlsreverse polarity protection diodezener diode voltage regulator for IC...and that's about it. Rather than blindly copy the schematic without fully understanding it I was hoping somebody could let me know of any alterations I may need to make, etc.The main concerns I have are:If I use a 5V regulator and IC (instead of the 3.6V one in the schematic) will I need to adjust the values of any passives in the Q4 area?Similarly, if I remove the reverse polarity diode (e.g. use a polarized connector or NPN MOSFET), will any of the passives around Q1/Q2/Q3 need to be adjusted for the ~0.7V higher voltage?Is there anything obvious in the schematic that is limiting the output power? The range of the transmitter I have is fairly short...Thanks!

author: Unknown
author_rank: Unknown
timestamp: November 02, 2017, 10:17:38 am
content: Hi, that looks like a real cheapo 27Mhz AM transmitter typical of toy cars. The PPM will be generated by the IC which looks to be a micro, most likely a general purpose one which has been custom labeled and pre-programmed for the manufacturer. If you really want to build your own then I would remove R10 and ZD1 and replace them with a 3.3V linear regulator, then just use a 3.3V micro. If you really want to use a 5V micro then do the same with a 5V regulator, you may need to increase R22 / R26 in this case if it over modulates. Removing the reverse polarity diode should have no effect (the battery voltage will vary more than 0.7V over it's life).If you are making your own PCB then copy the layout of the area around Q1 Q2 Q3 as closely as possible, all the way to the aerial. Make sure your aerial is the same length and diameter as the original. The output power is limited by the design (simple single output stage) and the low capacity of the battery.The range will be limited by:The very low output power of the transmitterThe short aerials usedThe likely poor receiver in the car

author: MWisBest
author_rank: Contributor
timestamp: November 02, 2017, 09:59:30 pm
content: I'm just used to using microcontrollers at 5V for some reason. I can certainly do 3.3V. Thanks for the heads up on R10, forgot zeners like this use a resistor. I was planning on making a PCB for the Q1 Q2 Q3 area but didn't realize the layout was important.So regarding the output power, if I used a higher voltage (say ~12V from 3 li-ion cells) would it/could it boost the power? Or would it just fry some components

author: Unknown
author_rank: Unknown
timestamp: November 03, 2017, 01:32:27 pm
content: Using 3.3V micros is no different really, just ensure your programmer is set up for them. I would use one here as it matches the original.Increasing the supply will likely give slightly more power and I doubt it will fry but probably won't give you any significant gains. If you really want more you could try adding a second parallel output transistor. Duplicate the parts connected to Q2 (R20 C10 R19) with a second transistor and connect the collectors together, the base resistor to the same place as Q2. You will likely need to add some more turns to the secondary of T1 to peak the output. If you are going to try this then build your circuit with just the one transistor populated first and get it working as close to the original as you can, then add in the second transistor. I wouldn't try to push too much out of this design as there is very little output filtering.Layout at these frequencies & power levels isn't hyper critical, but it can cause problems so if you have a working one to copy then do that, it's one less thing to worry about.

author: MWisBest
author_rank: Contributor
timestamp: November 03, 2017, 03:17:14 pm
content: Quote from: Z80 on November 03, 2017, 01:32:27 pmThanks, that all sounds very good! I'm not trying to make a CB radio or something, just want to run the car down the street a little further. I do have a working one to copy, the working one is actually through-hole instead of SMD like the schematic which is a bonus for me.Is there a different transistor I should try using? The working one uses 2SC1815Y's. I'm guessing at 27MHz it's not too critical, but since I'm going to have to order some things anyway I thought I should ask.

author: Unknown
author_rank: Unknown
timestamp: November 06, 2017, 08:31:55 am
content: At these frequencies and power levels any general jellybean transistor should be ok. BC547 or something like that.