  Currently several low-frequency experiments are being planned to study the
nature of the first stars using the redshifted 21-cm signal from the cosmic
dawn and epoch of reionization. Using a one-dimensional radiative transfer
code, we model the 21-cm signal pattern around the early sources for different
source models, i.e., the metal-free Population III (PopIII) stars, primordial
galaxies consisting of Population II (PopII) stars, mini-QSOs and high-mass
X-ray binaries (HMXBs). We investigate the detectability of these sources by
comparing the 21-cm visibility signal with the system noise appropriate for a
telescope like the SKA1-low. Upon integrating the visibility around a typical
source over all baselines and over a frequency interval of 16 MHz, we find that
it will be possible to make a $\sim 9-\sigma$ detection of the isolated sources
like PopII galaxies, mini-QSOs and HMXBs at $z \sim 15$ with the SKA1-low in
1000 hours. The exact value of the signal to noise ratio (SNR) will depend on
the source properties, in particular on the mass and age of the source and the
escape fraction of ionizing photons. The predicted SNR decreases with
increasing redshift. We provide simple scaling laws to estimate the SNR for
different values of the parameters which characterize the source and the
surrounding medium. We also argue that it will be possible to achieve a SNR
$\sim 9$ even in the presence of the astrophysical foregrounds by subtracting
out the frequency-independent component of the observed signal. These
calculations will be useful in planning 21-cm observations to detect the first
sources.
