What does inverted triangle in this schematic mean?

1. This circuit is intended to pick up radio signals. Radio signals don't do well on solderless breadboards. There is capacitance between rows on the breadboard that will tend to "absorb" your radio signals. I'd solder the circuit together.
2. The circuit is intended to pick up radio signals. Radio signals don't do well with long wires. Long wires act like inductors that will tend to block your radio signals. The leads on the parts are long enough. Solder the circuit together with the shortest connections you can make. You should be able to make everything with just 1 cm long leads.
3. The "inverted triangle" is normally "signal ground" in a circuit. In a radio receiver, it should really be "earth ground" - that is, it should be connected to a long wire driven into the dirt of the earth. Since you will want this thing to be portable, it should be enough if you make the ground wire something you can touch when you carry the device. Hold the device, touch the ground wire, wave the device around looking for strong signals.
4. The capacitors in the schematic diagram are 1 nanofarad. If you are using ceramic capacitors, they will be marked "102" That's 10 times 100 (two zeros) picofarads = 1000 picofarads = 1 nanofarad.

You can build the circuit on a piece of perfboard, or you can build it by just soldering the components together.

• The diodes may be rather sensitive to heat when soldering. Clip an alligator clip to the leads of the diodes (right up by the body of the diode) while soldering to keep the heat away from the diode. Too much heat can damage a diode. It might look OK, but not work.
• Small ceramic capacitors are rather fragile. Be careful when bending the leads of the capacitors - hold the leads with a needle nosed pliers right up by the body of the capacitor while bending the leads of the capacitors. That way, the pliers take the mechanical force of the bend rather than the body of the capacitor.

I wouldn't make any bets on the frequency range of the radio signals you can receive, or the sensitivity of the device.

• It'll probably react to quite weak sources at low frequencies and short distances. (Microprocessors and a interference from other electronic devices.)
• It'll probably react to much higher frequencies but at higher signal levels at short distances. A portable two way radio (walkie talkie) will probably cause a strong reaction.
• It might react to cellphone signals (the phone transmitting) at short distance, and maybe WiFi or Bluetooth. It will depend on the length if the connections in the circuit, and how well the diodes work at very high frequencies. Try it. Might work, might not.

The inverted triangle symbol represents 'signal ground'.

A signal ground serves as a return path for signals and power and also serves as a common reference for incoming and outgoing signals.

In the circuit being referred, it is the common terminating point for D1, C2 and the 10kΩ resistor.

The components may be soldered, 'point-to-point' on a perforated board, using short leads. A PCB may also be used.

Capacitor values are marked on them using the 3-digit value code. For a capacitor marked '102', the first significant digit is '1', the second significant digit '0' and the number of zeroes that follow '2'. The value of a capacitor marked '102' would be 1000pF or 0.001μF.

The two 0.001μF capacitors, you are to use, would be marked '102'.

I'm just getting started myself, but I think the inverted triangle (the one at the bottom?) is Signal Ground (not power ground). But what you connect this to I'm not sure. I've read about using a metal cold water pipe in your building, or a long metal rod driven into the actual ground.

I'm not sure about the rest, sorry. However, be careful not to overheat the diodes. Apparently it's a good idea to use alligator clips or the like as a heatsink to protect them when you solder.

Good luck!