\documentclass[border=2pt]{standalone}\usepackage[american]{circuitikz}\begin{document}\begin{circuitikz}[scale=1, transform shape]
%--- Place the lamp at top-left\node[lamp, label=above:4V Light] (lamp) at (0,3) {};
%--- Place an NPN transistor below the lamp\node[npn, label=left:BC547] (Q1) at (0,1.5) {};
%--- Connect the lamp to the transistor collector\draw (lamp.base) -- (Q1.collector);
%--- Battery in the middle\node[battery1, rotate=270,
label={[shift={(0,0.3)}]above:3.7V Battery}] (Bat) at (2,2) {};
%--- Transistor emitter to battery negative\coordinate (neg) at (0,0);
\draw (Q1.emitter) -- (neg);
\draw (Bat.negative) -- ++(-0.5,0) |- (neg);
%--- Battery positive to lamp\draw (Bat.positive) -- ++(0.5,0) |- (lamp.top);
%--- Solar panel on the right (just drawn as a rectangle)\draw (4,3) rectangle (6,0);
% Label inside the rectangle\node at (5,1.5) {Solar Panel};
% Mark the + and - leads on left edge of panel\draw (4,3) node[left] {$+$};
\draw (4,0) node[left] {$-$};
%--- Connect battery + to solar panel +\draw (Bat.positive) -- (4,3);
%--- Diode and resistor from solar panel – to battery negative\draw (4,0)
to [D, l^=1N4007] (5.5,0)
to [R, l^=1k$\Omega$] (neg);
\end{circuitikz}\end{document}
Explanation of the main pieces:
The lamp (lamp) is placed at the top left.
The BC547 transistor (npn) is placed below the lamp.
The battery is drawn with \node[battery1, rotate=270]{} so that its positive terminal is up and negative is down.
The solar panel is simply a rectangle labeled "Solar Panel" with its + and -- terminals on the left edge.
A diode (1N4007) in series with a 1 k Ω resistor connects from the panel's negative side back to the circuit negative (transistor emitter and battery negative).
