If a circuit diagram calls for a certain capacitor you could use a capacitor with a slightly higher farad rating and it would work the same right?

 

Depends entirely on the application, some are very value specific.
Max.

 

If the capacitor sets the frequency of oscillation or a time delay, then the capacitor directly influences that parameter.

If the capacitor is used as filter, bypass, or coupling cap, then it is not so critical.

 

A circuit I'm using calls for a couple capacitors that I can't seem to find. Since most capacitors have a tolerance of +-20%, as long as the capacitor is within a couple pF it should be fine right?

 

A 10pF capacitor with 20% tolerance maybe.

Otherwise your assumption is incorrect.

Show us the circuit with the capacitor in question and then we can discuss the tolerance required.

 

The ones I can't find are the 203pF, 151pF, 103pF, 102pF, 222pF, 104pF, and 501pF

 

Whoever chose those component values must be nuts. Just round the last digit to zero.

Let's do one at a time. First the transmitter:

Item | Value | Substitute | Comment
C1|203pF|200-100,000pF|non-critical filter cap
C2|47μF|10μF-100μF|non-critical supply filter cap
C3|47|47|non-critical
C4|68|68|non-critical
C5|203|200-100,000|non-critical filter
C6|68|68|non-critical
C7|203|200-1000|non-critical HF filter
C8|151|150|non-critical
C9|151|150|tuned tank circuit will depend on L3 in any case
C10|103|100|tuned tank circuit will depend on L3 in any case

 

Thanks Mr chips! That's pretty much what I thought. The only ones that are important to have really close capacitance to what's written are the ones from L3 to ground.

 

There is a strong possibility that someone transcribed the values incorrectly from another schematic:

Item | Marking | Value | Comment
C1|203|20nF|non-critical filter cap
C2|47μF|10μF-100μF|non-critical supply filter cap
C3|47|47pF|non-critical
C4|68|68pF|non-critical
C5|203|20nF|non-critical filter
C6|68|68pF|non-critical
C7|203|20nF|non-critical HF filter
C8|151|150pF|non-critical
C9|151|150pF|tuned tank circuit will depend on L3 in any case
C10|103|10nF|this one does not seem right

 

BTW, for a 27MHz crystal you want to choose one of the following frequencies used for 27MHz RC band:

• 26.995 MHz -- Channel 1 (Brown)
• 27.045 MHz -- Channel 2 (Red)
• 27.095 MHz -- Channel 3 (Orange)
• 27.145 MHz -- Channel 4 (Yellow)
• 27.195 MHz -- Channel 5 (Green)
• 27.255 MHz -- Channel 6 (Blue)

 

I ordered one of these-
http://www.taydaelectronics.com/cry...0-mhz-27-mhz-crystal-hc-49-s-low-profile.html

 

As far as I know 27.000MHz is not a legal frequency.
Operating at this frequency will interfere with CB Channel 4 operating at 27.005MHz.
There is a 10kHz separation between legal alloted channels in the 27MHz band which has been allocated for CB and RC.

 

Well it's +-20ppm.

 

Well it's +-20ppm.

That doesn't matter. You need to understand frequency spread caused by modulation.

 

Read the bill of materials again. I do believe the third numeral is a power of 10.

 

Ok MrChips, sorry. I understand what you're saying now.

 

Shouldn't there be a receiver crystal in the receiver circuit? Or does this circuit not need one?

 

If anyone has any suggestions on a completely different transmitter/receiver feel free to tell me. I just need at least 4 channels.

 

Get yourself some of these. Do not get the ones without the encoder/decoder chips.

I am using 4 of the 315MHz transmitters and one of the receivers. I mounted the receiver inside a weatherproof fullsize 1/2 wavelength vertically-polarized Dipole antenna which is mounted on the outside front of my metal hangar building. I am able to open/close a swing gate, two garage doors, and the aircraft hangar door from a distance of about 100m from the inside of of a car.



It is all about how good the receive antenna is, and where it is mounted....

 

Both the transmitter and receiver should have a crystal to set them at the same frequency right?
And is this a better circuit for what I'm trying to do?